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Seed to Leaf, by Jane H. Newell, Illustrated by H. P. Symmes
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Title: Outlines of Lessons in Botany, Part I; From Seed to Leaf
Author: Jane H. Newell
Release Date: January 16, 2004 [eBook #10726]
Language: English
***START OF THE PROJECT GUTENBERG EBOOK OUTLINES OF LESSONS IN BOTANY,
PART I; FROM SEED TO LEAF***
E-text prepared by Juliet Sutherland, Keren Vergon, Leonard D. Johnson,
and Project Gutenberg Distributed Proofreaders
OUTLINES OF LESSONS IN BOTANY.
PART I.: FROM SEED TO LEAF
FOR THE USE OF TEACHERS, OR MOTHERS STUDYING WITH THEIR CHILDREN.
BY
JANE H. NEWELL.
ILLUSTRATED BY H.P. SYMMES
1888.
PART I
TABLE OF CONTENTS
I. PLANTS AND THEIR USES
1. Food
2. Clothing
3. Purification of the Air
4. Fuel
II. SEEDLINGS
1. Directions for raising in the Schoolroom
2. Study of Morning-Glory, Sunflower, Bean, and Pea
3. Comparison with other Dicotyledons
4. Nature of the Caulicle
5. Leaves of Seedlings
6. Monocotyledons
7. Food of Seedlings
III. ROOTS
1. Study of the Roots of Seedlings
2. Fleshy Roots
3. Differences between Stem and Root
4. Root-hairs
5. Comparison of a Carrot, an Onion, and a Potato
IV BUDS AND BRANCHES
1. Horsechestnut
Magnolia
Lilac
Beech
American Elm
Balm of Gilead
Tulip-tree
Cherry
Red Maple
Norway Spruce
2. Vernation
3. Phyllotaxy
V STEMS
1. Forms
2. Movements
3. Structure
VI LEAVES
1. Forms and Structure
2. Descriptions
3. Transpiration
4. Assimilation
5. Respiration
PREFACE.
In this study, as in all scientific teaching, the teacher's aim should
be to foster in his pupils the power of careful observation and clear
expression. The actual amount of knowledge gained at school must needs be
small, and often quickly forgotten, but the habit of right study is an
invaluable possession.
The former method of teaching Botany was confined almost wholly to dry,
technical classification. The pupil learned to find the name and order of
a plant, but its structure, its habits, its life in short, were untouched
by him. We know now that Nature is the best text-book. The pupil should
first ask his questions of her and try to interpret her answers; then he
may learn with profit what those who better understand her speech have to
tell him.
This method of teaching, however, requires much, very much, of the
teacher. He must be himself intelligent, well trained, and able to give
time to the preparation of his lessons. It seems to us, who are but
amateurs, as if it were impossible to teach thus without a thorough
comprehension of the whole field. Our own ignorance oppresses us so much
that we feel tempted to say that we cannot attempt it. But if the work of
leading children to observe the wonders about them is to be done at all,
it must be done by us, who are not masters of our subject, and we must
find out for ourselves how we can best accomplish this result, since we
have so little to guide us.
It is with the hope that the experience of one who has tried to do
this with some fair amount of success may be of use to other puzzled
experimenters, that I venture to write out some outlines of lessons in
Botany for beginners.
The method of beginning with the simpler forms of life is one that appeals
to the scientific tendencies of the day. It seems logical to begin with
lower forms and work up to the higher. But this method is only suitable
for mature minds. We do not teach a child English by showing him the
sources of the language; he learns it by daily use. So also the beginning
of the study of any Natural Science by the young should be the observation
of the most obvious things about them, the things which they can see, and
handle, and experiment upon naturally, without artificial aids. Therefore
this book concerns itself only with the Flowering Plants.
The author believes that the simplest botanical study should afford the
means of identifying plants, as a large part of the student's pleasure in
the science will be the recognition of the things about him. The present
volume affords the basis for future classification, which Part II, on
flowers, will develop. It is, doubtless, as good a way, perhaps the best,
to begin with a single plant, and study root, stem, leaves, and flowers
as belonging to a whole, but the problem is complicated by practical
difficulties. In our climate there are but two months of the school year
when flowers are easily obtained. On the other hand, the material for
these lessons can be got throughout the winter, and the class, well
trained in methodical work, will begin the study of flowers at the season
when every day brings some fresh wonder of beauty.
The author will receive gladly any criticisms or suggestions.
JANE H. NEWELL.
175 Brattle St., Cambridge
INTRODUCTION.
The lessons here outlined are suitable for children of twelve years of
age, and upwards. For younger pupils they would require much adaptation,
and even then they would not be so good as some simpler method, such as
following the growth of one plant, and comparing it with others at every
step. The little ones profit most by describing the very simple things
that they see, without much reference to theories.
The outlines follow the plan of Dr. Gray's First Lessons and How Plants
Grow, and are intended to be used in connection with either of those
books. The necessary references will be found at the end of every section.
The book contains also references to a course of interesting reading in
connection with the subjects of the lessons.
The lessons may begin, like the text-books, with the subject of
Germination, if the seeds are planted before they are required for use,
but it is generally preferable to use the first recitation with the class
for planting the seeds, in order to have them under the direct care of the
pupils. Some general talks about plants are therefore put at the beginning
to occupy the time until the seedlings are ready for study.
Some Nasturtiums (_Tropæolum majus_) and Morning-Glories should be planted
from the first in boxes of earth and allowed to grow over the window, as
they are often used for illustrations.
I.
PLANTS AND THEIR USES.[1]
[Footnote 1: This section may be omitted, and the lessons begun with
Seedlings, if the teacher prefer.]
What is Botany? The pupils are very apt to say at first that it is
learning about _flowers_. The teacher can draw their attention to the fact
that flowers are only a part of the plant, and that Botany is also the
study of the leaves, the stem, and the root. Botany is the science of
_plants_. Ask them what the Geranium is. Tell them to name some other
plants. The teacher should keep a few growing plants in the schoolroom for
purposes of illustration.
Ask them what else there is in the world besides plants. By this question
the three kingdoms, animal, vegetable, and mineral, are brought up. It
will give occasion for a discussion of the earth and what it contains, the
mountains, formed of rocks and soil, the plants growing on the earth,
and the animals that inhabit it, including man. Let them name the three
kingdoms with some example of each. Which of these kingdoms contain living
things? The words _organic_ and _inorganic_ can be brought in here. An
_organ_ ([Greek: Ergon], meaning work) is any part that does a special
work, as the leaves, the stem of a plant, and the eye, the ear of animals.
An _organism_ is a living being made up of such organs. The inorganic
world contains the mineral kingdom; the organic world includes the
vegetable and animal kingdoms.
One's aim in these lessons should always be to tell the pupils as little
as possible. Try to lead them to think out these things for themselves.
Ask them how plants differ from animals. They will say that plants are
fixed to one place, while animals can move about; that plants have no will
or consciousness, and that animals have. These answers are true when we
compare the higher animals with plants, but the differences become lost as
we descend in the scale and approach the border land where botanist and
zoologist meet on a common ground. Sea-anemones are fixed to the rock on
which they grow, while some of the lower plants are able to move from
place to place, and it is hardly safe to affirm that a jelly-fish is more
conscious of its actions than is a Sensitive Plant, the leaves of which
close when the stem is touched.
There is no real division between animals and plants. We try to classify
the objects about us into groups, according to the closeness of their
relationships, but we must always remember that these hard lines are ours,
not Nature's. We attempt, for purposes of our own convenience, to divide a
whole, which is so bound together that it cannot be separated into parts
that we can confidently place on different sides of a dividing line.
1. _Plants as Food-Producers_.--The chief distinguishing characteristic of
plants is one that the pupils may be led to think out for themselves by
asking them what animals feed upon. To help them with this, ask them what
they had for breakfast. Oatmeal is mentioned, perhaps. This is made from
oats, which is a plant. Coffee and tea, bread made from wheat, potatoes,
etc., all come from plants.[1] Beef, butter and milk come from the cow,
but the cow lives upon grass. The plant, on the other hand, is nourished
upon mineral or inorganic matter. It can make its own food from the soil
and the air, while animals can only live upon that which is made for
them by plants. These are thus the link between the mineral and animal
kingdoms. Ask the scholars if they can think of anything to eat or drink
that does not come from a plant. With a little help they will think of
salt and water. These could not support life. So we see that animals
receive all their food through the vegetable kingdom. One great use of
plants is that they are _food-producers_.
[Footnote 1: Reader in Botany, for use in Schools. Selected and adapted
from well-known authors. Ginn & Co., Boston, New York and Chicago, 1889.
I. Origin of Cultivated Plants.]
This lesson may be followed by a talk on food and the various plants used
for food.[2]
[Footnote 2: The Flour Mills of Minneapolis: Century Magazine, May, 1886.
Maize: Popular Science News, Nov. and Dec., 1888.]
2. _Clothing_.--Plants are used for clothing. Of the four great clothing
materials, cotton, linen, silk, and woollen, the first two are of
vegetable, the last two of animal origin. Cotton is made from the hairs of
the seed of the cotton plant.[1] Linen is made of the inner fibre of
the bark of the flax plant. It has been cultivated from the earliest
historical times.
[Footnote 1: Reader in Botany. II. The Cotton Plant.]
3. _Purification of the Air_.--The following questions and experiments are
intended to show the pupils, first, that we live in an atmosphere, the
presence of which is necessary to support life and combustion (1) and (2);
secondly, that this atmosphere is deprived of its power to support life
and combustion by the actions of combustion (2), and of respiration (3);
thirdly, that this power is restored to the air by the action of plants
(4).
We have the air about us everywhere. A so-called empty vessel is one
where the contents are invisible. The following experiment is a good
illustration of this.
(1) Wrap the throat of a glass funnel with moistened cloth or paper so
that it will fit tightly into the neck of a bottle, and fill the funnel
with water. If the space between the funnel and the bottle is air-tight,
the water will not flow into the bottle.
[Illustration: FIG. 1.]
Do not explain this in advance to the pupils. Ask them what prevents
the water from flowing into the bottle. If they are puzzled, loosen the
funnel, and show them that the water will now flow in. In the first case,
as the air could not escape, the water could not flow in; in the second,
the air was displaced by the heavier water.
Ask the pupils why the air in a crowded room becomes so difficult to
breathe. Could a person live if he were shut up in an air-tight room for a
long time? Fresh air is necessary to life. The teacher may explain that it
is the oxygen in the air that supports life. Air is composed one-fifth of
this gas and four-fifths of nitrogen. The gases are mixed and the nitrogen
simply dilutes the oxygen, as it were.
Fresh air is necessary to support combustion as well as life. Ask them why
we put out a fire by throwing a blanket or a rug over it. The following
experiment illustrates this.
(2) Take a small, wide-mouthed bottle, covered with a card or cork. To
this cover fasten a piece of bent wire with a taper on the end. Light the
taper and lower it into the jar. It will burn a few seconds and then go
out. Raise and light it again, and it will be extinguished as soon as it
is plunged into the bottle. This shows that the oxygen of the air is used
up by burning substances, as it is by breathing animals.
[Illustration: FIG. 2.]
The following experiment shows that fire will not burn in an atmosphere of
gas from our lungs.
(3) Fill a bottle with gas by breathing into it through a bit of glass
tubing, passed through a card or cork, and reaching to the bottom of the
bottle. The bottle will be dimmed with moisture, showing the presence of
aqueous vapor. A lighted match plunged into the bottle will be immediately
extinguished. A better way, which, however, takes some skill in
manipulation, is to fill the bottle with water, cover it with a flat piece
of glass, and invert the bottle in a dish of water, taking care that no
air bubbles enter. Then, through a bit of glass tubing, blow into the
bottle till the water is expelled. Cover the mouth with the glass under
water, and holding it tightly down, invert the bottle quickly. Set it
down, light a match, take away the glass, and at the same instant plunge
in the match. If no air has been allowed to enter, the match will go out
at once. No animal could live in an atmosphere which could not support
combustion.
From these experiments the pupils have seen that the life-sustaining
quality of the air is used up by combustion and respiration. To bring in
the subject of purification by plants, ask them why all the oxygen in
the world is not exhausted by the people and the fires in it. After the
subject has been explained, the following experiment can be prepared and
put aside till the next lesson.
(4) Fill two bottles with air from the lungs, as in (3) having previously
introduced a cutting from a plant into one of the bottles. Allow them to
stand in the sun for a day or two. Then test both bottles with a burning
match. If properly done, the result will be very striking. The end of
the cutting should be in the water of the dish. This experiment will not
succeed excepting with bottles such as are used for chemicals, which have
their mouths carefully ground. Common bottles allow the air to enter
between the bottle and the glass.[1]
[Footnote 1: See note on page 13.]
[Illustration: FIG. 3.]
4. _Fuel_.--Light a match and allow it to burn until half charred. Blow it
out gently, so as to leave a glowing spark. When this spark goes out it
will leave behind a light, gray ash. We have to consider the flame, the
charred substance, and the ash.
Flame is burning gas. In all ordinary fuels, carbon and hydrogen, in
various combinations and free, make the principal part. The first effect
of the heat is to set free the volatile compounds of carbon and hydrogen.
The hydrogen then begins to unite with the oxygen of the air, forming
water, setting free the carbon, which also unites with oxygen, forming
carbonic acid gas. The burning gases cause the flame. The following
experiment will illustrate this.
[Illustration: Fig. 4.]
(5) Fit a test-tube with a tight cork, through which a bit of glass
tubing, drawn out into a jet, is passed, the tubing within being even with
the cork. Place some bits of shaving in the tube, cork it, and make the
cork perfectly air-tight by coating it with bees wax or paraffine. Heat
the test-tube gently over an alcohol lamp. The wood turns black, and vapor
issues from the jet, which may be lighted (Fig. 4). Care should be taken
to expel all the air before lighting.
(6) That the burning hydrogen forms water by uniting with the oxygen of
the air, may be shown by holding a cold glass tumbler over the jet, or
over any flame. The glass will be dimmed by drops of moisture.
The charred part of the wood is charcoal, which is one form of carbon.
Our ordinary charcoal is made by driving off all the gases from wood, by
burning it under cover where only a little air can reach it. The volatile
gases burn more readily than the carbon, and are the first substances to
be driven off, so that the carbon is left behind nearly pure. In the same
way we have driven off all the gases from the half-burned match and left
the carbon. The teacher should have a piece of charcoal to show the
pupils. It still retains all the markings of the wood.
If the combustion is continued, the carbon also unites with the oxygen of
the air, till it is all converted into carbonic acid gas. This was the
case with the match where we left the glowing spark. The gray ash that was
left behind is the mineral matter contained in the wood.
(7) We can show that this gas is formed by pouring lime water into a
bottle in which a candle has been burned as in (2). The water becomes
milky from a fine white powder formed by the union of the carbonic acid
gas with the lime, forming carbonate of lime. This is a chemical test.
The wood of the match is plainly of vegetable origin; so also is the
charcoal, which is nearly pure carbon. Coal is also carbon, the remains of
ancient forests, from which the gases have been slowly driven off by heat
and pressure. All the common fuels are composed principally of carbon and
hydrogen. When these elements unite with oxygen, carbonic acid gas and
water are formed.[1]
[Footnote 1: [Transcriber's Note: This note is missing from original
text.]]
(8) The same products are formed by respiration. We breathe out carbonic
acid gas and water from our lungs. Breathe on a cold glass. It is bedewed
exactly as it is by the candle flame. Breathe through a bit of glass
tubing into a bottle of lime water. It becomes milky, showing the presence
of carbonic acid gas. Why is this?
Every act or thought is accompanied by a consumption of material in the
body, which thus becomes unfit for further use. These waste substances,
composed chiefly of carbon and hydrogen, unite with oxygen breathed in
from the air, forming carbonic acid gas and water, which are breathed
out of the system. The action is a process of slow combustion, and it is
principally by the heat thus evolved that the body is kept warm. As we are
thus constantly taking oxygen from the air, a close room becomes unfit to
live in and a supply of fresh air is indispensable. The cycle of changes
is completed by the action of plants, which take in carbonic acid gas, use
the carbon, and return most of the oxygen to the atmosphere.
APPARATUS FOR EXPERIMENTS.[1]
[Footnote 1: The glass apparatus required, including an alcohol lamp, may
be obtained for one dollar by sending to the Educational Supply Co., No. 6
Hamilton Place, Boston.]
Two small wide-mouthed bottles. A narrow-necked bottle. A glass funnel. A
bit of bent glass-tubing. A bit of straight glass-tubing. A flat piece of
glass. A test-tube, with jet. An alcohol lamp. A bent wire with taper.
A card. A slip of a plant. A dish and pitcher of water. Beeswax or
paraffine. Shavings. Lime water. Matches.
_Gray's First Lessons. Revised edition_. Sect. XVI, 445-7, 437.
_How Plants Grow_. Chap. III, 279-288.
II.
SEEDLINGS.
1. _Directions for raising in the Schoolroom_.--The seeds should be
planted in boxes tilled with clean sand. Plates or shallow crockery pans
are also used, but the sand is apt to become caked, and the pupils are
likely to keep the seeds too wet if they are planted in vessels that
will not drain. The boxes should be covered with panes of glass till the
seedlings are well started, and should be kept at a temperature of from
65° to 70° Fahr. It is very important to keep them covered while the seeds
are germinating, otherwise the sand will be certain to become too dry if
kept in a sufficiently warm place. Light is not necessary, and in winter
time the neighborhood of the furnace is often a very convenient place
to keep them safe from frost. They should not be in the sun while
germinating. When the first sprouts appear above the ground let another
set be planted, and so on, till a series is obtained ranging from plants
several inches high to those just starting from the seed. The seeds
themselves should be soaked for a day and the series is then ready
for study. The time required for their growth varies according to the
temperature, moisture, etc. Dr. Goodale says they should be ready in ten
days.[1]
[Footnote 1: Concerning a few Common Plants, by G.L. Goodale, Boston, D.C.
Heath & Co. This little book, which is published, in pamphlet form, for
fifteen cents, will be found exceedingly useful.]
I have never been able to raise them so quickly in the schoolroom, nor
have the pupils to whom I have given them to plant done so at home.
Generally, it is three weeks, at least, before the first specimens are as
large as is desirable.
Germinating seeds need warmth, moisture and air. The necessary conditions
are supplied in the very best way by growing them on sponge, but it would
be difficult to raise enough for a large class in this manner. Place a
piece of moist sponge in a jelly-glass, or any glass that is larger at the
top, so that the sponge may not sink to the bottom, and pour some water
into the glass, but not so much as to touch the sponge. The whole should
be covered with a larger inverted glass, which must not be so close as
to prevent a circulation of air. The plants can thus be watched at every
stage and some should always be grown in this way. The water in the
tumbler will keep the sponge damp, and the roots, after emerging from
the sponge, will grow well in the moist air. Seeds can also be grown on
blotting paper. Put the seeds on several thicknesses of moist blotting
paper on a plate, cover them with more moist paper, and invert another
plate over them, taking care to allow the free entrance of air.
If possible, it is by far the best way to have the seeds growing in the
schoolroom, and make it a regular custom for the pupils to observe them
every morning and take notes of their growth.
These lessons on seeds are suitable for pupils of every age, from adults
to the youngest children who go to school. The difference should be only
in the mode of treatment; but the same principles should be brought out,
whatever the age and power of comprehension of the pupil.
For these lessons the following seeds should be planted, according to the
above directions:
Morning-Glory, Sunflower or Squash, Bean, Pea, Red Clover, Flax, Corn,
Wheat, and Oats.[1] If they can be procured plant also acorns, Pine-seeds,
Maple-seeds, and horsechestnuts.
[Footnote 1: A package of these seeds may be obtained for fifty cents,
from Joseph Breck & Son, Boston, Mass. They will be sent by mail, postage
paid.]
2. _Study of Morning-Glory, Sunflower, Bean, and Pea_.--For reasons
hereafter given, I consider the Morning-Glory the best seedling to begin
upon. Having a series, as above described, before them, the pupils should
draw the seedlings. When the drawings are made, let them letter alike the
corresponding parts, beginning with the plantlet in the seed, and using
new letters when a new part is developed. The seed coats need not be
lettered, as they do not belong to the plantlet.
[Illustration: FIG. 5.--Germination of Morning Glory, _a_, caulicle; _b_,
cotyledons; _c_, plumule; _d_, roots.]
[Illustration: FIG. 6.--Germination of Sunflower.]
After drawing the Morning-Glory series, let them draw the Sunflower or
Squash in the same way, then the Bean, and finally the Pea. Let them write
answers to the following questions:
MORNING-GLORY.[1]
[Footnote 1: It has been objected that the Morning-Glory seed is too small
to begin upon. If the teacher prefer, he may begin with the Squash, Bean,
and Pea. The questions will require but little alteration, and he can take
up the Morning-Glory later.]
Tell the parts of the Morning-Glory seed.
What part grows first?
What becomes of the seed-covering?
What appears between the first pair of leaves?
Was this to be seen in the seed?
How many leaves are there at each joint of stem after the first pair?
How do they differ from the first pair?
SUNFLOWER OR SQUASH.
What are the parts of the seed?
What is there in the Morning-Glory seed that this has not?
How do the first leaves change as the seedling grows?
BEAN.
What are the parts of the seed?
How does this differ from the Morning-Glory seed?
How from the Sunflower seed?
How do the first pair of leaves of the Bean change as they grow?
How many leaves are there at each joint of stem?[1]
[Footnote 1: There are two simple leaves at the next node to the
cotyledons; after these there is one compound leaf at each node.]
How do they differ from the first pair?
PEA.
What are the parts of the seed? Compare it with the Morning-Glory,
Sunflower, and Bean.
How does it differ in its growth from the Bean?
What have all these four seeds in common?
[Illustration: FIG. 7.--Germination of Pea. _a_, caulicle; _b_,
cotyledons; _c_, plumule; _d_, roots.]
[Illustration: FIG. 8.--Germination of Bean.]
What has the Morning-Glory seed that the others have not?
What have the Bean and Pea that the Morning-Glory has not?
How does the Pea differ from all the others in its growth?
What part grows first in all these seeds?
From which part do the roots grow?
What peculiarity do you notice in the way they come up out of the
ground?[1]
[Footnote 1: This question refers to the arched form in which they come
up. In this way the tender, growing apex is not rubbed.]
The teacher must remember that, unless the pupils have had some previous
training, they will first have to learn to use their eyes, and for this
they will need much judicious help. They should be assisted to see what is
before them, not told what is there. It is absolutely necessary that these
questions should be thoroughly understood and correctly answered before
any conclusions are drawn from them. For this purpose abundant material is
indispensable. It is better not to attempt these lessons on seeds at
all, unless there is material enough for personal observation by all the
pupils.
After this preliminary work has been done, the names of the parts can
be given to the pupils. They may be written under each drawing
thus,--A=Caulicle;[1] B=Cotyledons; C=Roots; D=Plumule. The whole plantlet
in the seed is the _embryo_ or _germ_, whence the sprouting of seeds is
called _germination_.
[Footnote 1: The term radicle is still in general use. The derivation
(little root) makes it undesirable. Dr. Gray has adopted caulicle (little
stem) in the latest edition of his text-book, which I have followed. Other
writers use the term hypocotyl, meaning under the cotyledons.]
I consider this the best order to study the seeds because in the
Morning-Glory the cotyledons are plainly leaves in the seed; and in the
Squash or Sunflower[2] the whole process is plainly to be seen whereby
a thick body, most unlike a leaf, becomes an ordinary green leaf with
veins.[3] In the Sunflower the true leaves are nearly the same shape as
the cotyledons, so that this is an especially good illustration for the
purpose. Thus, without any hint from me, my pupils often write of the
Bean, "it has two thick leaves and two thin leaves." In this way the Bean
and Pea present no difficulty. The cotyledons in the first make apparently
an unsuccessful effort to become leaves, which the second give up
altogether.
[Footnote 2: The large Russian Sunflower is the best for the purpose.]
[Footnote 3: These lessons are intended, as has been said, for children
over twelve years of age. If they are adapted for younger ones, it is
especially important to begin with a seed where the leaf-like character
of the cotyledons is evident, or becomes so. Maple is excellent for the
purpose. Morning-Glory is too small. Squash will answer very well. I think
it characteristic of the minds of little children to associate a term with
the first specimen to which it is applied. If the term cotyledon be given
them first for those of the Bean and Pea they will say when they come to
the Morning-Glory, "but those are _leaves_, not cotyledons. Cotyledons are
large and round." It will be very difficult to make them understand that
cotyledons are the first seed-leaves, and they will feel as if it were a
forced connection, and one that they cannot see for themselves.]
The teacher's object now is to make the pupils understand the meaning of
the answers they have given to these questions. In the first place, they
should go over their answers and substitute the botanical terms they have
just learned for the ones they have used.
COMPARISON OF THE PARTS OF THE SOAKED SEEDS.
_Morning-Glory_. A seed covering. Some albumen. Two cotyledons. A
caulicle.
_Sunflower_. An outer covering.[1] An inner covering. Two cotyledons. A
caulicle.[2]
[Footnote 1: The so-called seed of Sunflower is really a fruit. The outer
covering is the wall of the ovary, the inner the seed-coat. Such closed,
one-seeded fruits are called akenes.]
[Footnote 2: The plumule is sometimes visible in the embryo of the
Sunflower.]
_Bean_. A seed covering. Two cotyledons. A caulicle. A plumule.
_Pea_. The same as the Bean.
They have also learned how the first leaves in the last three differ from
those of the Morning-Glory, being considerably thicker in the Sunflower,
and very much thicker in the Bean and Pea. Why should the Morning-Glory
have this jelly that the others have not? Why do the first leaves of the
Sunflower change so much as the seedling grows? What becomes of their
substance? Why do those of the Bean shrivel and finally drop off? By this
time some bright pupil will have discovered that the baby-plant needs food
and that this is stored around it in the Morning-Glory, and in the leaves
themselves in the others. It is nourished upon this prepared food, until
it has roots and leaves and can make its own living. The food of the
Morning-Glory is called _albumen_; it does not differ from the others in
kind, but only in its manner of storage.[1]
[Footnote 1: Reader in Botany. III. Seed-Food.]
Also the questions have brought out the fact that the Bean and Pea
have the plumule ready formed in the seed, while the Morning-Glory and
Sunflower have not. Why should this be? It is because there is so much
food stored in the first two that the plumule can develop before a root is
formed, while in the others there is only nourishment sufficient to enable
the plantlet to form its roots. These must make the second leaves by their
own labor.
3. _Comparison with other Dicotyledons_.--The pupils should now have other
seeds to compare with these four. Let them arrange Flax, Four o-clock,
Horsechestnut, Almond, Nasturtium, Maple-seeds, etc., under two heads.
_Seeds with the Food stored _Seeds with the Food stored
outside the plantlet in the embryo itself
(Albuminous)_. (Exalbuminous)_.
Flax. Four-o'clock. Acorn. Horsechestnut. Almond.
Morning-Glory. Maple. Sunflower. Squash.
Bean. Pea. Nasturtium.
They may also be divided into those with and without the plumule.
_Without Plumule_. _With Plumule_.
Flax. Maple. Sunflower. Acorn. Horsechestnut.
Four-o'clock. Almond. Bean. Pea.
Morning-Glory. Squash. Nasturtium.
Those with plumules will be seen to have the most abundant nourishment. In
many cases this is made use of by man.
These last can be again divided into those in which the cotyledons come up
into the air and those where they remain in the ground.
_In the Air_. _In the Ground_.
Bean. Almond. Squash. Acorn. Horsechestnut.
Pea. Nasturtium.
In the latter the cotyledons are so heavily gorged with nourishment that
they never become of any use as leaves. As Darwin points out, they have
a better chance of escaping destruction by animals by remaining in the
ground.
The cotyledons are very good illustrations of the different uses to which
a single organ may be put, and the thorough understanding of it will
prepare the pupils' minds for other metamorphoses, and for the theory that
all the various parts of a plant are modified forms of a very few members.
4. _Nature of the Caulicle_.--Probably some of the pupils will have called
the caulicle the root. It is, however, of the nature of stem. The root
grows only at the end, from a point just behind the tip; the stem
elongates throughout its whole length. This can be shown by marking the
stem and roots of a young seedling with ink. India ink must be used, as
common ink injures the plants. Dip a needle in the ink and prick a row
of spots at equal distances on a young root. Corn is very good for this
purpose, but Morning-Glory or Bean is better for experiments on the
stem. The plants should then be carefully watched and the changes in
the relative distance of the spots noted. The experiment is very easily
conducted with the seedlings growing on sponge, with their roots in the
moist air of the tumbler, as before described.
Dr. Goodale says of this experiment,--"Let a young seedling of corn be
grown on damp paper in the manner described in No. 1,[1] and when the
longest root is a few centimetres long let it be marked very carefully by
means of India ink, or purple ink, put on with a delicate camel's-hair
pencil just one centimetre apart. Plants thus marked are to be kept under
favorable conditions with respect to moisture and warmth, so that growth
will be as rapid as possible. The marks on the older part of the root
will not change their relative distance, but the mark at the tip will be
carried away from the one next it, showing that the growth has taken place
only at this point. Such experiments as the one described are perfectly
practicable for all classes of pupils except the very youngest. How far
the details of these experiments should be suggested to the pupils, or
rather how far they should be left to work out the problem for themselves,
is a question to be settled by the teacher in each case. The better plan
generally is to bring the problem in a very clear form before the whole
class, or before the whole school, and ask whether anybody can think of a
way in which it can be solved; for instance, in this case how can it be
found out whether roots grow only at their tip or throughout their whole
length. If the way is thought out by even a single pupil the rest will be
interested in seeing whether the plan will work successfully."
[Footnote 1: Concerning a Few Common Plants, page 25.]
I have been more successful in pricking the roots than in marking them
with a brush.
The caulicle can be proved by the manner of its growth to be of the nature
of stem, not root. The main root grows from its naked end. Roots can also
grow from the sides of the caulicle, as in Indian Corn. In this, it acts
precisely as does the stem of a cutting. It can be prettily shown with the
seedlings by breaking off a bean at the ground and putting the slip in
water. It will throw out roots and the pupil will readily understand that
the caulicle does the same thing.
Darwin has made very interesting experiments on the movements of
seedlings. If the teacher wishes to repeat some of the experiments he will
find the details very fully given in "The Power of Movement of Plants."[1]
The pupils can observe in their growing seedlings some of the points
mentioned and have already noticed a few in their answers. They have said
that the caulicle was the part to grow first, and have spoken of the
arched form of the young stem. Their attention should also be drawn to the
root-hairs, which are well seen in Corn, Wheat, and Oats. They absorb the
liquid food of the plants. A secondary office is to hold the seed firmly,
so that the caulicle can enter the ground. This is shown in Red Clover,
which may be sown on the surface of the ground. It puts out root-hairs,
which attach themselves to the particles of sand and hold the seed. These
hairs are treated more fully in the lessons on roots.
[Footnote 1: The Power of Movement in Plants. By Charles Darwin. London.
John Murray, 1880.]
[Footnote 1: Reader in Botany. IV. Movements of Seedlings.]
5. _Leaves of Seedlings_.--Coming now to the question as to the number of
leaves at each joint of the stem, the Morning-Glory, Sunflower, and Bean
will present no difficulty, but probably all the pupils will be puzzled by
the Pea. The stipules, so large and leaf-like, look like two leaves,
with a stem between, bearing other opposite leaves, and terminating in a
tendril, while in the upper part it could not be told by a beginner which
was the continuation of the main stem. For these reasons I left this out
in the questions on the Pea, but it should be taken up in the class. How
are we to tell what constitutes a single leaf? The answer to this question
is that buds come in the _axils_ of single leaves; that is, in the inner
angle which the leaf makes with the stem. If no bud can be seen in the
Pea, the experiment may be tried of cutting off the top of the seedling
plant. Buds will be developed in the axils of the nearest leaves, and it
will be shown that each is a compound leaf with two appendages at its
base, called stipules, and with a tendril at its apex. Buds can be forced
in the same way to grow from the axils of the lower scales, and even from
those of the cotyledons, and the lesson may be again impressed that organs
are capable of undergoing great modifications. The teacher may use his own
judgment as to whether he will tell them that the tendril is a modified
leaflet.
[Illustration: FIG. 9. 1. Grain of Indian Corn. 2. Vertical section,
dividing the embryo, _a_, caulicle: _b_, cotyledon; _c_, plumule. 3.
Vertical section, at right angles to the last.]
6. _Monocotyledons_.--These are more difficult. Perhaps it is not worth
while to attempt to make the pupils see the embryo in Wheat and Oats. But
the embryo of Indian Corn is larger and can be easily examined after long
soaking. Removing the seed-covering, we find the greater part of the seed
to be albumen. Closely applied to one side of this, so closely that it
is difficult to separate it perfectly, is the single cotyledon. This
completely surrounds the plumule and furnishes it with food from the
albumen. There is a line down the middle, and, if we carefully bend back
the edges of the cotyledon, it splits along this line, showing the
plumule and caulicle within. The plumule consists of successive layers of
rudimentary leaves, the outer enclosing the rest (Fig. 10, 1, _c_). The
latter is the first leaf and remains undeveloped as a scaly sheath (Fig.
10, 2, _c_). In Wheat and Oats the cotyledon can be easily seen in the
largest seedlings by pulling off the dry husk of the grain. The food will
he seen to have been used up.
[Illustration: FIG. 10. 1. Germination of Indian corn. 2. Same more
advanced. _a_, caulicle; _c_1, first leaf of the plumule, sheathing the
rest; _c_2, second leaf; _c_3, third leaf of the plumule; _d_, roots.]
The series of Corn seedlings, at least, should be drawn as before and
the parts marked, this time with their technical terms. The following
questions should then be prepared.
CORN.
What are the parts of the seed?
Compare these parts with the Morning-Glory, Sunflower, Bean, and Pea.
Where is the food stored?
How many cotyledons have Corn, Wheat, and Oats?
How many have Bean, Pea, Morning-Glory, and Sunflower?
Compare the veins of the leaves of each class and see what difference you
can find.
This will bring up the terms dicotyledon and monocotyledon. _Di_ means
two, _mono_ means one. This difference in the veins, netted in the first
class, parallel in the second, is characteristic of the classes. Pupils
should have specimens of leaves to classify under these two heads.
Flowering plants are divided first into these two classes, the
Dicotyledons and the Monocotyledons.
If Pine-seeds can be planted, the polycotyledonous embryo can also be
studied.
7. _Food of seedlings_.--The food of the Wheat seedling may be shown in
fine flour. [1]"The flour is to be moistened in the hand and kneaded until
it becomes a homogeneous mass. Upon this mass pour some pure water and
wash out all the white powder until nothing is left except a viscid lump
of gluten. This is the part of the crushed wheat-grains which very closely
resembles in its composition the flesh of animals. The white powder washed
away is nearly pure wheat-starch. Of course the other ingredients, such as
the mineral matter and the like, might be referred to, but the starch at
least should be shown. When the seed is placed in proper soil, or upon a
support where it can receive moisture, and can get at the air and still be
warm enough, a part of the starch changes into a sort of gum, like that on
postage stamps, and finally becomes a kind of sugar. Upon this sirup the
young seedling feeds until it has some good green leaves for work, and as
we have seen in the case of some plants it has these very early."
[Footnote 1: Concerning a Few Common Plants, page 18.]
The presence of starch can be shown by testing with a solution of iodine.
Starch is turned blue by iodine and may thus be detected in flour, in
seeds, in potatoes, etc.
After all this careful experimental work the subject may be studied in the
text-book and recited, the recitation constituting a thorough review of
the whole.
A charming description of the germination of a seed will be found in the
Reader. V. The Birth of Picciola.
_Gray's Lessons_. Sect. II, 8-14. III. _How Plants Grow_. Sect. I, 22, 23.
II.
III
ROOTS.
This subject can be treated more conveniently while the young seedlings
are still growing, because their roots are very suitable for study. It
seems best, therefore, to take it up before examining the buds.
1. _Study of the Roots of Seedlings_.--One or two of the seedlings should
be broken off and the slips put into a glass of water. They will be
studied later. Bean and Sunflower are the best for the purpose.
Begin by telling the pupils to prepare for their first lesson a
description of the roots of their seedlings. Those grown on sponge or
paper will show the development of the root-hairs, while those grown on
sand are better for studying the form of the root. Give them also some
fleshy root to describe, as a carrot, or a radish; and a spray of English
Ivy, as an example of aërial roots.
Throughout these lessons, the method is pursued of giving pupils specimens
to observe and describe before teaching them botanical terms. It is better
for them to name the things they see than to find examples for terms
already learned. In the first case, they feel the difficulty of expressing
themselves and are glad to have the want of exact terms supplied. This
method is discouraging at first, especially to the younger ones; but,
with time and patience, they will gradually become accustomed to describe
whatever they can see. They have, at any rate, used their eyes; and,
though they may not understand the real meaning of anything they have
seen, they are prepared to discuss the subject intelligently when they
come together in the class. If they will first write out their unassisted
impressions and, subsequently, an account of the same thing after they
have had a recitation upon it, they will be sure to gain something in the
power of observation and clear expression. It cannot be too strongly
urged that the number of facts that the children may learn is not of the
slightest consequence, but that the teacher should aim to cultivate the
quick eye, the ready hand, and the clear reason.
The root of the Morning-Glory is _primary_; it is a direct downward growth
from the tip of the caulicle. It is about as thick as the stem, tapers
towards the end, and has short and fibrous branches. In some plants the
root keeps on growing and makes a _tap-root_; in the Bean, it soon becomes
lost in the branches. These are all simple, that is, there is but one
primary root. Sometimes there are several or many, and the root is then
said to be _multiple_. The Pumpkin is an example of this. The root of
the Pea is described in the older editions of Gray's Lessons as being
multiple, but it is generally simple. Indian Corn, also, usually starts
with a single root, but this does not make a tap-root, and is soon
followed by many others from any part of the caulicle, or even from the
stem above, giving it the appearance of having a multiple root.
The root of the Radish is different from any of these; it is _fleshy_.
Often, it tapers suddenly at the bottom into a root like that of
the Morning-Glory with some fibres upon it. It is, in fact, as the
Morning-Glory would be if the main root were to be thickened up by
food being stored in it. It is a primary tap-root. The radish is
_spindle-shaped_, tapering at top and bottom, the carrot is _conical_, the
turnip is called _napiform_; some radishes are shaped like the turnip.
The aërial roots of the English Ivy answer another purpose than that of
giving nourishment to the plant. They are used to support it in climbing.
These are an example of _secondary_ roots, which are roots springing
laterally from any part of the stem. The Sweet Potato has both fleshy and
fibrous roots and forms secondary roots of both kinds every year.[1] Some
of the seedlings will probably show the root-hairs to the naked eye. These
will be noticed hereafter.
[Footnote 1: Gray's Lessons, p. 35, Fig. 86.]
[Illustration: FIG. 11.--1. Tap-root. 2. Multiple root of Pumpkin. 3.
Napiform root of Turnip. 4. Spindle-shaped root of Radish. 5. Conical root
of Carrot. 6. Aërial roots of Ivy.]
It is my experience that pupils always like classifying things under
different heads, and it is a good exercise. The following table may be
made of the roots they have studied, adding other examples. Dr. Gray says
that ordinary roots may be roughly classed into fibrous and fleshy.[1]
Thomé classes them as woody and fleshy.[2]
[Footnote 1: Gray's Lessons, p. 34.]
[Footnote 2: Text-book of Structural and Physiological Botany. Otto Thomé.
Translated and edited by Alfred W. Bennett, New York. John Wiley and Sons.
1877. Page 75.]
ROOTS.
|
------------------------------------------
| |
_Primary_. _Secondary_.
| |
-------------------------------- |
| | |
_Fibrous_. _Fleshy_. Roots of cuttings
| Aërial roots.
------------------- Sweet potatoes.[3]
| |
_Simple_. _Multiple_. _Simple_.
Morning Glory. Pumpkin Carrot.
Sunflower. Radish.
Pea. Turnip.
Bean. Beet.
Corn. Corn.
[Footnote 3: The Irish potato will very likely be mentioned as an example
of a fleshy root. The teacher can say that this will be explained later.]
2. _Fleshy Roots_.--The scholars are already familiar with the storing
of food for the seedling in or around the cotyledons, and will readily
understand that these roots are storehouses of food for the plant. The
Turnip, Carrot, and Beet are _biennials_; that is, their growth is
continued through two seasons. In the first year, they make a vigorous
growth of leaves alone, and the surplus food is carried to the root in the
form of a syrup, and there stored, having been changed into starch, or
something very similar. At the end of the first season, the root is filled
with food, prepared for the next year, so that the plant can live on its
reserve fund and devote its whole attention to flowering. These roots
are often good food for animals. There are some plants that store their
surplus food in their roots year after year, using up in each season the
store of the former one, and forming new roots continually. The Sweet
Potato is an example of this class. These are _perennials_. The food in
perennials, however, is usually stored in stems, rather than in roots, as
in trees. _Annuals_ are generally fibrous-rooted, and the plant dies after
its first year. The following experiment will serve as an illustration of
the way in which the food stored in fleshy roots is utilized for growth.
Cut off the tapering end of a carrot and scoop out the inside of the
larger half in the form of a vase, leaving about half of the flesh behind.
Put strings through the upper rim, fill the carrot cup with water, and
hang it up in a sunny window. Keep it constantly full of water. The
leaf-buds below will put forth, and grow into leafy shoots, which, turning
upwards, soon hide the vase in a green circle. This is because the dry,
starchy food stored in the carrot becomes soft and soluble, and the supply
of proper food and the warmth of the room make the leaf-buds able to grow.
It is also a pretty illustration of the way in which stems always grow
upward, even though there is enough light and air for them to grow
straight downwards. Why this is so, we do not know.
3. _Differences between the Stem and the Root.--_Ask the pupils to tell
what differences they have found.
_Stems_. _Roots_.
Ascend into the air. Descend into the ground.
Grow by a succession of similar Grow only from a point
parts, each part when young just behind the tip.
elongating throughout.
Bear organs. Bear no organs.
There are certain exceptions to the statement that roots descend into the
ground; such as aërial roots and parasitic roots. The aërial roots of the
Ivy have been mentioned. Other examples of roots used for climbing are
the Trumpet Creeper _(Tecoma radicans)_, and the Poison Ivy _(Rhus
Toxicodendron)_. Parasitic roots take their food ready-made from the
plants into which they strike. The roots of air-plants, such as certain
orchids, draw their nourishment from the air.
The experiment of marking roots and stem has been already tried, but it
should be repeated. Repetition of experiments is always desirable, as it
fixes his conclusions in the pupil's mind. The stem grows by a succession
of similar parts, _phytomera_, each part, or _phyton_, consisting of node,
internode, and leaf. Thus it follows that stems must bear leaves. The
marked stems of seedlings show greater growth towards the top of the
growing phyton. It is only young stems that elongate throughout. The older
parts of a phyton grow little, and when the internode has attained a
certain length, variable for different stems and different conditions, it
does not elongate at all.
The root, on the contrary, grows only from a point just behind the tip.
The extreme tip consists of a sort of cap of hard tissue, called the
root-cap. Through a simple lens, or sometimes with the naked eye, it can
be distinguished in most of the roots of the seedlings, looking like a
transparent tip. "The root, whatever its origin in any case may be, grows
in length only in one way; namely, at a point just behind its very
tip. This growing point is usually protected by a peculiar cap, which
insinuates its way through the crevices of the soil. If roots should grow
as stems escaping from the bud-state do,--that is, throughout their whole
length--they would speedily become distorted. But, since they grow at the
protected tips, they can make their way through the interstices of soil,
which from its compactness would otherwise forbid their progress."[1]
[Footnote 1: Concerning a few Common Plants, p. 25.]
The third difference is that, while the stem bears leaves, and has buds
normally developed in their axils, roots bear no organs. The stem,
however, especially when wounded, may produce buds anywhere from the
surface of the bark, and these buds are called _adventitious_ buds. In the
same manner, roots occasionally produce buds, which grow up into leafy
shoots, as in the Apple and Poplar.[1]
[Footnote 1: See Gray's Structural Botany, p. 29.]
It should be made perfectly clear that the stem is the axis of the plant,
that is, it bears all the other organs. Roots grow from stems, not steins
from roots, except in certain cases, like that of the Poplar mentioned
above. This was seen in the study of the seedling. The embryo consisted of
stem and leaves, and the roots were produced from the stem as the seedling
grew.
For illustration of this point, the careful watching of the cuttings
placed in water will be very instructive. After a few days, small, hard
lumps begin to appear under the skin of the stem of the broken seedling
Bean. These gradually increase in size until, finally, they rupture the
skin and appear as rootlets. Roots are always thus formed under the outer
tissues of the stem from which they spring, or the root from which they
branch. In the Bean, the roots are in four long rows, quartering the stem.
This is because they are formed in front of the woody bundles of the stem,
which in the seedling Bean are four. In the Sunflower the roots divide the
circumference into six parts. In some of my cuttings of Beans, the stem
cracked in four long lines before the roots had really formed, showing the
parenchyma in small hillocks, so to speak. In these the gradual formation
of the root-cap could be watched throughout, with merely a small lens. I
do not know a better way to impress the nature of the root on the pupil's
mind. These forming roots might also be marked very early, and so be shown
to carry onward their root-cap on the growing-point.
4. _Root-hairs_. These are outgrowths of the epidermis, or skin of the
root, and increase its absorbing power. In most plants they cannot be seen
without the aid of a microscope. Indian Corn and Oats, however, show them
very beautifully, and the scholars have already noticed them in their
seedlings. They are best seen in the seedlings grown on damp sponge. In
those grown in sand, they become so firmly united to the particles of
soil, that they cannot be separated, without tearing the hairs away from
the plant. This will suggest the reason why plants suffer so much from
careless transplanting.
The root-hairs have the power of dissolving mineral matters in the soil
by the action of an acid which they give out. They then absorb these
solutions for the nourishment of the plant. The acid given out was first
thought to be carbonic acid, but now it is supposed by some experimenters
to be acetic acid, by others to vary according to the plant and the time.
The action can be shown by the following experiment, suggested by Sachs.
[Illustration: Fig. 12. I. Seedling of _Sinapis alba_ showing root-hairs.
II. Same, showing how fine particles of sand cling to the root-hairs.
(Sachs.)]
Cover a piece of polished marble with moist sawdust, and plant some seeds
upon it. When the seedlings are somewhat grown, remove the sawdust, and
the rootlets will be found to have left their autographs behind. Wherever
the roots, with their root-hairs have crept, they have eaten into the
marble and left it corroded. The marks will become more distinct if the
marble is rubbed with a little vermilion.
In order that the processes of solution and absorption may take place, it
is necessary that free oxygen should be present. All living things must
have oxygen to breathe, and this gas is as needful for the germination of
seeds, and the action of roots and leaves, as it is for our maintenance of
life. It is hurtful for plants to be kept with too much water about their
roots, because this keeps out the air. This is the reason why house-plants
are injured if they are kept too wet.
A secondary office of root-hairs is to aid the roots of seedlings to enter
the ground, as we have before noticed.
The root-hairs are found only on the young parts of roots. As a root grows
older the root-hairs die, and it becomes of no further use for absorption.
But it is needed now for another purpose, as the support of the growing
plant. In trees, the old roots grow from year to year like stems, and
become large and strong. The extent of the roots corresponds in a general
way to that of the branches, and, as the absorbing parts are the young
rootlets, the rain that drops from the leafy roof falls just where it is
needed by the delicate fibrils in the earth below.[1]
[Footnote 1: Reader in Botany. VI. The Relative Positions of Leaves and
Rootlets.]
5. _Comparison of a Carrot, an Onion, and a Potato_.--It is a good
exercise for a class to take a potato, an onion, and a carrot or radish to
compare, writing out the result of their observations.
The carrot is a fleshy root, as we have already seen. The onion consists
of the fleshy bases of last year's leaves, sheathed by the dried remains
of the leaves of former years, from which all nourishment has been drawn.
The parallel veining of the leaves is distinctly marked. The stem is a
plate at the base, to which these fleshy scales are attached. In the
centre, or in the axils of the scales, the newly-forming bulbs can be
seen, in onions that are sprouting. If possible, compare other bulbs, as
those of Tulip, Hyacinth, or Snowdrop, and the bulb of a Crocus, in which
the fleshy part consists of the thickened base of the stem, and the leaves
are merely dry scales. This is called a _corm_.
The potato is a thickened stem. It shows itself to be a stem, because it
bears organs. The leaves are reduced to little scales (eyelids), in the
axils of which come the buds (eyes). The following delightful experiment
has been recommended to me.
In a growing potato plant, direct upwards one of the low shoots and
surround it with a little cylinder of stiff carpet paper, stuffed with
sphagnum and loam. Cut away the other tuber-disposed shoots as they
appear. The enclosed shoot develops into a tuber which stands more or less
vertical, and the scales become pretty little leaves. Removing the paper,
the tuber and leaves become green, and the latter enlarge a little. A
better illustration of the way in which organs adapt themselves to their
conditions, and of the meaning of morphology, could hardly be found.
_Gray's First Lessons_. Sect. v, 65-88. _How Plants Grow_. Chap. I, 83-90.
IV.
BUDS AND BRANCHES.
1. There is an astonishing amount to be learned from naked branches,
and, if pursued in the right way, the study will be found exceedingly
interesting. Professor Beal, in his pamphlet on the New Botany,[1] says:--
"Before the first lesson, each pupil is furnished or told where to procure
some specimen for study. If it is winter, and flowers or growing plants
cannot be had, give each a branch of a tree or shrub; this branch may be
two feet long. The examination of these is made during the usual time for
preparing lessons, and not while the class is before the teacher. For the
first recitation each is to tell what he has discovered. The specimens are
not in sight during the recitation. In learning the lesson, books are not
used; for, if they are used, no books will contain a quarter of what the
pupil may see for himself. If there is time, each member of the class is
allowed a chance to mention anything not named by any of the rest. The
teacher may suggest a few other points for study. The pupils are not told
what they can see for themselves. An effort is made to keep them working
after something which they have not yet discovered. If two members
disagree on any point, on the next day, after further study, they are
requested to bring in all the proofs they can to sustain their different
conclusions. For a second lesson, the students review the first lesson,
and report on a branch of a tree of another species which they have
studied as before. Now they notice any point of difference or of
similarity. In like manner new branches are studied and new comparisons
made. For this purpose, naked branches of our species of elms, maples,
ashes, oaks, basswood, beech, poplars, willows, walnut, butternut,
hawthorns, cherries, and in fact any of our native or exotic trees or
shrubs are suitable. A comparison of the branches of any of the evergreens
is interesting and profitable. Discoveries, very unexpected, are almost
sure to reward a patient study of these objects. The teacher must not
think time is wasted. No real progress can be made, till the pupils begin
to learn to see; and to learn to see they must keep trying to form the
habit from the very first; and to form the habit they should make the
study of specimens the main feature in the course of training."
[Footnote 1: The New Botany. By W.J. Beal. Philadelphia, C.H. Marot, 814
Chestnut St., 1882. Page 5.]
HORSECHESTNUT (_Æsculus Hippocastanum_).
We will begin with the study of a branch of Horsechestnut.[1] The pupils
should examine and describe their specimens before discussing them in the
class-room. They will need some directions and hints, however, to enable
them to work to any advantage. Tell them to open both large and small
buds. It is not advisable to study the Horsechestnut bud by cutting
sections, as the wool is so dense that the arrangement cannot be seen in
this way. The scales should be removed with a knife, one by one, and the
number, texture, etc., noted. The leaves and flower-cluster will remain
uncovered and will be easy to examine. The gum may be first removed by
pressing the bud in a bit of paper. The scholars should study carefully
the markings on the stem, in order to explain, if possible, what has
caused them. The best way to make clear the meaning of the scars is to
show them the relation of the bud to the branch. They must define a bud.
Ask them what the bud would have become the next season, if it had been
allowed to develop. It would have been a branch, or a part of one. A bud,
then, is an undeveloped branch. They can always work out this definition
for themselves. Conversely, a branch is a developed bud, or series of
buds, and every mark on the branch must correspond to something in the
bud. Let them examine the specimens with this idea clearly before their
minds. The lesson to prepare should be to write out all they can observe
and to make careful drawings of their specimens. Ask them to find a way,
if possible, to tell the age of the branch.
[Footnote 1: The pupils should cut their names on their branches and keep
them. They will need them constantly for comparison and reference.]
At the recitation, the papers can be read and the points mentioned
thoroughly discussed. This will take two lesson-hours, probably, and the
drawing may be left, if desired, as the exercise to prepare for the second
recitation.
[1]The buds of Horsechestnut contain the plan of the whole growth of the
next season. They are scaly and covered, especially towards the apex, with
a sticky varnish. The scales are opposite, like the leaves. The outer
pairs are wholly brown and leathery, the succeeding ones tipped with
brown, wherever exposed, so that the whole bud is covered with a thick
coat. The inner scales are green and delicate, and somewhat woolly,
especially along the lapping edges. There are about seven pairs of
scales. The larger terminal buds have a flower-cluster in the centre, and
generally two pairs of leaves; the small buds contain leaves alone, two or
three pairs of them. The leaves are densely covered with white wool, to
protect them from the sudden changes of winter. The use of the gum is to
ward off moisture. The flower-cluster is woolly also.
[Footnote 1: All descriptions are made from specimens examined by me.
Other specimens may differ in some points. Plants vary in different
situations and localities.]
The scars on the stem are of three kinds, leaf, bud-scale, and
flower-cluster scars. The pupils should notice that the buds are always
just above the large triangular scars. If they are still in doubt as to
the cause of these marks, show them some house-plant with well-developed
buds in the axils of the leaves, and ask them to compare the position of
these buds with their branches. The buds that spring from the inner angle
of the leaf with the stem are _axillary_ buds; those that crown the stems
are _terminal_. Since a bud is an undeveloped branch, terminal buds carry,
on the axis which they crown, axillary buds give rise to side-shoots. The
leaf-scars show the leaf-arrangement and the number of leaves each year.
The leaves are opposite and each pair stands over the intervals of the
pair below. The same is observed to be true of the scales and leaves
of the bud.[1] All these points should be brought out by the actual
observation of the specimens by the pupils, with only such hints from the
teacher as may be needed to direct their attention aright. The dots on the
leaf-scar are the ends of woody bundles (fibro-vascular bundles) which, in
autumn, separated from the leaf. By counting these we can tell how many
leaflets there were in the leaf, three, five, seven, nine, or occasionally
six or eight.
[Footnote 1: Bud-scales are modified leaves and their arrangement is
therefore the same as the leaves. This is not mentioned in the study of
the Horsechestnut bud, because it cannot be proved to the pupils, but the
transition is explained in connection with Lilac, where it may be clearly
seen. The scales of the bud of Horsechestnut are considered to be
homologous with petioles, by analogy with other members of the same
family. In the Sweet Buckeye a series can be made, exhibiting the gradual
change from a scale to a compound leaf. See the Botanical Text-Book, Part
I, Structural Botany. By Asa Gray. Ivison, Blakeman, Taylor and Co., New
York, 1879. Plate 233, p. 116.]
[Illustration: FIG. 13.--Horsechestnut. I. Branch in winter state: _a_,
leaf-scars; _b_, bud-scars; _c_, flower-scars. 2. An expanding leaf-bud.
3. Same, more advanced.]
_The Bud Scale-Scars_. These are rings left by the scales of the bud and
may be seen in many branches. They are well seen in Horsechestnut. If the
pupils have failed to observe that these rings show the position of former
buds and mark the growth of successive years, this point must be brought
out by skilful questioning. There is a difference in the color of the more
recent shoots, and a pupil, when asked how much of his branch grew the
preceding season, will be able to answer by observing the change in color.
Make him see that this change corresponds with the rings, and he will
understand how to tell every year's growth. Then ask what would make the
rings in a branch produced from one of his buds, and he can hardly fail to
see that the scales would make them. When the scholars understand that the
rings mark the year's growth, they can count them and ascertain the age
of each branch. The same should be done with each side-shoot. Usually the
numbers will be found to agree; that is, all the buds will have the
same number of rings between them and the cut end of the branch, but
occasionally a bud will remain latent for one or several seasons and then
begin its growth, in which case the numbers will not agree; the difference
will be the number of years it remained latent. There are always many buds
that are not developed. "The undeveloped buds do not necessarily perish,
but are ready to be called into action in case the others are checked.
When the stronger buds are destroyed, some that would else remain dormant
develop in their stead, incited by the abundance of nourishment which the
former would have monopolized. In this manner our trees are soon reclothed
with verdure, after their tender foliage and branches have been killed by
a late vernal frost, or consumed by insects. And buds which have remained
latent for several years occasionally shoot forth into branches from the
sides of old stems, especially in certain trees."[1]
[Footnote 1: Structural Botany, p. 48.]
The pupils can measure the distance between each set of rings on the main
stem, to see on what years it grew best.
_The Flower-Cluster Scars_. These are the round, somewhat concave, scars,
found terminating the stem where forking occurs, or seemingly in the
axils of branches, on account of one of the forking branches growing more
rapidly and stoutly than the other and thus taking the place of the main
stem, so that this is apparently continued without interruption. If the
pupils have not understood the cause of the flower-cluster scars, show
them their position in shoots where they are plainly on the summit of the
stem, and tell them to compare this with the arrangement of a large
bud. The flower-cluster terminates the axis in the bud, and this scar
terminates a branch. When the terminal bud is thus prevented from
continuing its growth, the nearest axillary buds are developed.[1] One
shoot usually gets the start, and becomes so much stronger that it throws
the other to one side. The tendency of the Horsechestnut to have its
growth carried on by the terminal buds is so strong that I almost feel
inclined to say that vigorous branches are never formed from axillary
buds, in old trees, except where the terminal bud has been prevented from
continuing the branch. This tendency gives to the tree its characteristic
size of trunk and branches, and lack of delicate spray. On looking closely
at the branches also, they will be seen to be quite irregular, wherever
there has been a flower-cluster swerving to one side or the other.
[Footnote 1: The first winter that I examined Horsechestnut buds I found,
in many cases, that the axillary shoots had from a quarter of an inch to
an inch of wood before the first set of rings. I could not imagine what
had formed this wood, and it remained a complete puzzle to me until the
following spring, when I found in the expanding shoots, that, wherever
a flower-cluster was present, there were one or two pairs of leaflets
already well developed in the axils, and that the next season's buds were
forming between them, while the internodes of these leaflets were making
quite a rapid growth. Subsequently, I found the leaflets also in the buds
themselves. I found these leaflets developed on the tree only in the
shoots containing flower-clusters, where they would be needed for the
future growth of the branches. I suppose the reason must be that the
flower-cluster does not use all the nourishment provided and that
therefore the axillary buds are able to develop. It would be interesting
to know what determines the stronger growth of the one which eventually
becomes the leader.]
There is one thing more the pupils may have noticed. The small round dots
all over the young stem, which become long rifts in the older parts, are
breaks in the epidermis, or skin of the stem, through which the inner
layers of bark protrude. They are called lenticels. They provide a passage
for gases in and out of the stem. In some trees, as the Birch, they are
very noticeable.
After discussing the subject thoroughly in the class-room, the pupils
should rewrite their papers, and finally answer the following questions,
as a species of review. I have thus spent three recitations on the
Horsechestnut. The work is all so new, and, if properly presented,
so interesting, that a good deal of time is required to exhaust its
possibilities of instruction. If the teacher finds his scholars wearying,
however, he can leave as many of the details as he pleases to be treated
in connection with other branches.
QUESTIONS ON THE HORSECHESTNUT.
How many scales are there in the buds you have examined?
How are they arranged?
How many leaves are there in the buds?
How are they arranged?
Where does the flower-cluster come in the bud?
Do all the buds contain flower-clusters?
What is the use of the wool and the gum?
Where do the buds come on the stem?
Which are the strongest?
How are the leaves arranged on the stem?
Do the pairs stand directly over each other?
What are the dots on the leaf-scars?
How old is your branch?
How old is each twig?
Which years were the best for growth?
Where were the former flower-clusters?
What happens when a branch is stopped in its growth by flowering?
What effect does this have on the appearance of the tree?
In some parts of the country the Horsechestnut is not so commonly planted
as in New England. In the southern states the Magnolia may be used in its
stead, but it is not nearly so simple an example of the main points to be
observed.[1]
[Footnote 1: Reader in Botany. VII. Trees in Winter.]
MAGNOLIA UMBRELLA.
The bud may be examined by removing the scales with a knife, as in
Horsechestnut, and also by cutting sections. The outer scales enfold the
whole bud, and each succeeding pair cover all within. They are joined,
and it is frequently difficult to tell where the suture is, though it can
generally be traced at the apex of the bud. On the back is a thick
stalk, which is the base of the leaf-stalk. Remove the scales by cutting
carefully through a single pair, opposite the leaf-stalk, and peeling
them off. The scales are modified stipules, instead of leaf-stalks, as in
Horsechestnut. The outer pair are brown and thick, the inner green, and
becoming more delicate and crumpled as we proceed toward the centre of the
bud. The leaves begin with the second or third pair of scales. The first
one or two are imperfect, being small, brown, and dry. The leaves grow
larger towards the centre of the bud. They are covered with short,
silky hairs, and are folded lengthwise, with the inner surface within
(_conduplicate_). In the specimens I have examined I do not see much
difference in size between the buds with flowers and those without. In
every bud examined which contained a flower, there was an axillary bud in
the axil of the last, or next to the last, leaf. This bud is to continue
the interrupted branch in the same way as in Horsechestnut.
There are from six to ten good leaves, in the buds that I have seen. Those
without flowers contain more leaves, as in Horsechestnut. In the centre of
these buds the leaves are small and undeveloped. The flower is very easy
to examine, the floral envelopes, stamens and pistils, being plainly
discernible. The bud may also be studied in cross-section. This shows the
whole arrangement. The plan is not so simple as in Horsechestnut, where
the leaves are opposite. The subject of leaf-arrangement should be passed
over until phyllotaxy is taken up.
The scars on the stem differ from Horsechestnut in having no distinct
bands of rings. The scales, being stipules, leave a line on each side of
the leaf-scar, and these are separated by the growth of the internodes.
In the Beech, the scales are also stipules; but, whereas in the Magnolia
there are only one or two abortive leaves, in the Beech there are eight or
nine pairs of stipules without any leaves at all. The rings thus become
separated in Magnolia, while in the Beech the first internodes are not
developed, leaving a distinct band of rings, to mark the season's growth.
The Magnolia is therefore less desirable to begin upon. The branches are
swollen at the beginning of a new growth, and have a number of leaf-scars
crowded closely together. The leaf-scars are roundish, the lower line more
curved. They have many dots on them. From each leaf-scar runs an irregular
line around the stem. This has been left by the stipules.
The flower-scar is on the summit of the axis, and often apparently in the
axil of a branch, as in Horsechestnut. Sometimes the nearest axillary bud
is developed; sometimes there are two, when the branch forks. The axillary
buds seldom grow unless the terminal bud is interrupted. The tree
therefore has no fine spray.
LILAC _(Syringa vulgaris_).
Ask the scholars to write a description of their branches and to compare
them with Horsechestnut. These papers should be prepared before coming
into the class, as before.
The buds are four-sided. The scales and leaves are opposite, as in
Horsechestnut. The outer pair sometimes have buds in their axils. Remove
the scales one by one with a knife, or better, with a stout needle. The
scales gradually become thinner as we proceed, and pass into leaves, so
that we cannot tell where the scales end and leaves begin. After about six
pairs are removed, we come, in the larger buds, to leaves with axillary
flower-clusters. The leaves grow smaller and the flower-clusters
larger till we come to the centre, where the axis is terminated by a
flower-cluster. There is a great difference in the buds on different
bushes and on shoots of the same bush, some being large, green, and easy
to examine, others small, hard, and dark-colored. It is better, of course,
to select as soft and large buds as possible for examination.
[Illustration: FIG. 14.--Lilac. I. Branch in winter state: _a_, leaf-scar;
_b_, bud-scar (reduced). 2. Same, less reduced. 3. Branch, with leaf-buds
expanded. 4. Series in a single bud, showing the gradual transition from
scales to leaves.]
That the scales are modified leaves is plainly shown by the gradual
transition they undergo, and also by the fact that buds are developed in
their axils. If any of these can be shown to the pupils, remind them of
the experiment where the top of a seedling Pea was cut off and buds forced
to develop in the axils of the lower scales.[1] The transition from scales
to leaves can be well studied by bringing branches into the house, where
they will develop in water, and towards spring may even be made to
blossom. Cherry, Apple, Forsythia, and other blossoming trees and shrubs
can be thus forced to bloom. Place the branches in hot water, and cut off
a little of their ends under water. If the water is changed every day,
and the glass kept near the register or stove, they will blossom out very
quickly. These expanded shoots may be compared with the buds. The number
of leaves in the bud varies.
[Footnote 1: See p. 31.]
The leaf-scars of Lilac are horseshoe-shaped and somewhat swollen. It can
often be plainly seen that the outer tissue of the stem runs up into the
scar. It looks as if there were a layer of bark, ending with the scar,
fastened over each side of the stem. These apparent layers alternate as
well as the scars. The epidermis, or skin of the leaves, is in fact always
continuous with that of the stem. There are no dots on the leaf-scars.
The rings are not nearly so noticeable as in Horsechestnut, but they can
be counted for some years back.
The flower-cluster can often be traced by a dried bit of stem remaining on
the branch.
The terminal bud in the Lilac does not usually develop, and the two
uppermost axillary buds take its place, giving to the shrub the forked
character of its branching. In all these bud studies, the pupil should
finish by showing how the arrangement of the buds determines the growth of
the branches.
QUESTIONS ON THE LILAC.
How do the scales differ from those of Horsechestnut?
How many scales and leaves are there?
How are they arranged?
Where does the flower-cluster come in the bud?
Do all the buds contain flower-clusters?
How does the arrangement of leaves and flower-clusters differ from that of
Horsechestnut?
How old is your branch?
Which buds develop most frequently?
How does this affect the appearance of the shrub?
COPPER BEECH (_Fagus sylvatica, var. purpurea_).
The buds are long and tapering, the scales thin and scarious, the outer
naked, the inner with long, silky hairs. Remove the scales one by one, as
in Lilac. The outer four or six pairs are so minute that the arrangement
is not very clear, but as we proceed we perceive that the scales are in
alternate pairs, as in Horsechestnut; that is, that two scales are exactly
on the same plane. But we have learned in the Lilac that the scales are
modified leaves, and follow the leaf-arrangement of the species. The
Beech is alternate-leaved, and we should therefore expect the scales to
alternate. The explanation is found as we go on removing the scales. At
the eighth or ninth pair we come upon a tiny, silky leaf, directly between
the pair of scales, and, removing these, another larger leaf, opposite the
first but higher up on the rudimentary stem, and so on, with the rest of
the bud. There are five or more leaves, each placed between a pair of
scales. Our knowledge of the parts of a leaf shows us at once that the
scales must be modified stipules, and that therefore they must be in
pairs.[1] Other examples of scales homologous with stipules are the
American Elm, Tulip-tree, Poplar and Magnolia. The leaves are plaited
on the veins and covered with long, silky hairs. The venation is very
distinct. The outer leaves are smaller and, on examining the branch, it
will be seen that their internodes do not make so large a growth as the
leaves in the centre of the bud.
[Footnote 1: See the stipules of the Pea, p. 31.]
[Illustration: FIG. 15.--Copper Beech. 1. Branch in winter state: _a_,
leaf-scar; _b_, bud-scar. 2. Branch, with leaf-buds expanding, showing the
plicate folding of the leaves.]
The leaf-scars are small, soon becoming merely ridges running half round
the stem.
The bud-rings are very plain and easily counted. For this reason, and
because it branches freely, it is a good tree for measurements of growth,
as is seen in the following tables. Nos. 1, 2, 3 and 4: were made by a
class of girls, from fourteen to sixteen, from a tree on my lawn. No. 5
was made by a pupil, whom I taught by correspondence, from a tree of the
same species in another town. No. 6 was made by myself from my own tree.
The measurements of the first four tables were somewhat revised by me, as
they were not perfectly accurate. The pupils should always be cautioned
to measure from the beginning of one set of rings to the beginning of the
next.[1]
[Footnote 1: Care must be taken to select branches well exposed to the
light. Of course there are many circumstances that may aid or hinder the
growth of any particular branch.]
NO. 1.
YEARS. GROWTH OF 1ST BRANCH. 2nd BRANCH. 3RD BRANCH 4TH BRANCH.
MAIN AXIS.
----------------------------------------------------------------
in.
'79 8-1/2 -- -- -- --
'80 4-1/2 2 1-7/8 -- --
'81 3-1/2 1-1/8 2-5/8 -- --
'82 6 5/8 4-1/4 5-7/8 --
'83 7-3/8 3-3/8 5-1/4 4 5-3/4
'84 2 1/2 3/4 3/8 5-3/8
'85 5/8 1/4 3/8 1/2 1
'86 5-5/8 7/8 4-3/8 3-1/8 5
NO. 2.
YEARS. GROWTH of 1ST 2nd 3RD 4TH 5TH
MAIN AXIS. BRANCH BRANCH BRANCH BRANCH BRANCH
----------------------------------------------------------------
in.
'79 8 -- -- -- -- -- --
'80 3-1/2 5-1/4 5-1/2 5-5/8 -- -- --
'81 4-3/4 3/4 1/2 2-1/2 2 -- --
'82 5-3/4 7/8 2 3/4 3/8 1/2 --
'83 5-1/4 4-3/4 5-1/2 4 3-1/4 2-3/8 1-3/4 --
'84 1/2 1 3/4 3/8 1 3/4 1 3/8
'85 2-3/4 1-3/4 4-3/8 3/4 3/4 2-1/8 3-1/4 1-1/4
'86 7-1/2 5-1/2 6-3/4 3 3 4-1/2 3-1/8 5
NO. 3.
YEARS. GROWTH of 1ST 2nd 3RD 4TH 5TH
MAIN AXIS. BRANCH BRANCH BRANCH BRANCH BRANCH
-----------------------------------------------------
in.
'80 8-1/4 -- -- -- -- --
'81 4-1/2 3-1/2 3-3/4 -- -- --
'82 5-1/2 3/4 1-1/2 1 -- --
'83 3-1/4 3-3/4 4-1/2 3/4 2 1-1/4
'84 5-1/2 1/2 3/4 1 1/2 3
'85 1/2 1-3/4 1/2 3/8 1 1/2
'86 4-1/4 3-3/8 2-3/8 1-1/4 2-1/4 1-1/2
NO. 4.
YEARS GROWTH 1ST 2nd 3RD 4TH
of MAIN BRANCH BRANCH BRANCH BRANCH
AXIS
-----------------------------------------
in.
'81 7-3/4 -- -- -- --
'82 8-3/4 6 6 -- --
'83 6-3/4 5-1/4 4 4-3/4 5-1/2
'84 4-1/2 5/8 1-5/8 2-1/4 3-1/4
'85 2 5/8 3/16 2 3/4
'86 10-3/4 1-3/4 1/4 7-1/4 3-1/2
NO. 4. (cont.)
YEARS 5TH 6TH 7TH 8TH 9TH
BRANCH BRANCH BRANCH BRANCH BRANCH
-----------------------------------
in.
'81 -- -- -- -- --
'82 -- -- -- -- --
'83 -- -- -- -- --
'84 3/4 2-1/2 -- -- --
'85 7/8 5/8 1/4 3/4 --
'86 4-3/4 6-3/8 1 2-1/4 6-1/2
NO. 5.
YEARS GROWTH 1ST 2nd 3RD 4TH 5TH 6TH
of MAIN BRANCH BRANCH BRANCH BRANCH BRANCH BRANCH
AXIS
-----------------------------------------------------
in.
'82 6-7/8 --- --- --- --- --- ---
'83 6-1/2 4-3/4 4-1/4 --- --- --- ---
'84 4-3/4 1/4 1-3/4 3-1/2 --- --- ---
'85 4-1/2 3/4 1 2-3/4 2-3/4 --- ---
'86 6-1/4 2-1/4 4-3/4 6-3/4 2-3/4 5-3/4 ---
'87 6-3/4 1-1/8 3-1/4 4 2-1/4 3 5-1/2
NO. 6.
YEARS MAIN 1ST 2ND 2ND 2ND 3RD 4TH
AXIS BRANCH BRANCH BRANCH BRANCH BRANCH BRANCH
-----------------------------------------------------
in. 1st 2nd
side side
'80 6-1/4 --- --- shoot. shoot. --- ---
'81 8-3/4 6-3/4 --- --- --- --- ---
'82 8-1/2 6-1/4 6-7/8 --- --- --- .
'83 4-3/4 1-1/2 2-3/8 --- --- 4 .
'84 3-1/2 3-1/8 5-1/8 --- --- 1-3/4 7/8
'85 4-1/2 3/8 4-3/4 2-1/4 --- 6 1
'86 6+ 6-3/4 12-1/8 5-1/2 10-1/2 8-7/8 5-1/8
'87 bough 2-1/2 8-3/4 4-1/4 4-1/4 4-6/8 3-3/4
broken.
One question brought up by these measurements is whether there is any
correspondence in growth between the main axis and its branches. It
appears in these tables that there is a general correspondence, in this
tree at least. In the recitation of the class, whose tables are given
above (Nos. 1, 2, 3 and 4), we took all the measurements of these four
branches for the year 1885 and added them. We did the same for 1886, and
compared the results. The total growth for 1885 was 31-15/16 inches; for
1886, leaving out the measurement of the twig whose entire growth was in
that year, 109-3/4 inches or nearly 3-1/2 times as much. The proportion
held in a general way throughout, there being only a single case of a
branch where the growth was greater in the first year.[1] But there is a
point that must not be overlooked in this connection. The branches of the
Beech seem to grow about equally well in the first, second, third, or any
succeeding year. In some trees, as the Ash, the axillary buds make a large
growth, and the succeeding terminal buds carry on the branch much more
slowly; in other trees, as the Cherry, a branch grows very slowly in the
first few years and then suddenly takes a start. These facts would appear
in tables of growth, made from branches of these trees, but the addition
of results for any particular year would have no significance.
[Footnote 1: The spring of 1880 was a remarkably early one. Thus I find in
my diary of that year the following entries:--
April 17. The red maples are in full bloom, the elms almost over. The
leaves of the Horsechestnut are quite large. The lilacs are nearly in
leaf. April 24. We went up to Waverley and found bloodroot up, spice bush
out, violets, dog-tooths and anemones, also caltha. April 28. All the
cherries are in full bloom. April 29. Picked an apple blossom in bud,
beautifully pink.
The season was nearly three weeks earlier than usual. 1885 on the other
hand was a late spring.]
In table No. 5, the addition of the measurements for 1885 and 1886 shows
the growth in the latter year to be about twice that of the former. This
branch came from a tree in another town. We have tried also to discover
whether the number of leaves each year has any relation to growth. I
cannot see that it has, but it requires many experiments to determine
these points. To study this, make tables of the number of leaves on the
branch each year. I think teachers would find it interesting to keep all
data of this kind of work done by their classes, with a view to tabulation
and comparison. The scholars themselves are exceedingly interested in
anything that partakes of the nature of an original investigation.[1]
[Footnote 1: The class, previously mentioned, were much interested in the
addition of their results. One of them asked me whether this subject of
measurements had been treated in any book. I replied that I had never seen
it mentioned. My attention was afterwards called to "What may be learned
from a Tree," by Prof. Harlan Couitas. D. Appleton & Co., New York, 1863.
I found, greatly to my surprise, that he had not only given diagrams of
growth, but that he also had selected a Copper Beech as his example.]
The leaf-arrangement of the Beech is alternate, on the one-half plan. The
small twigs turn upwards, so that all the spray is on the upper side,
giving a flat appearance to the branch.[1] This gives the leaves a better
exposure to the light. Both the terminal and axillary buds grow freely,
thus forming long, straight limbs, with many branches and much fine spray.
[Footnote 1: Phyllotaxy is treated later, by a comparison and study of
many branches, but the teacher can draw the pupils' attention to the fact
that each Beech leaf and twig is on exactly the opposite side of the
branch from the preceding one. This allows all the twigs to grow towards
one side of the branch, whereas in trees on the two-fifths plan, as the
Apple, Poplar, Oak, etc., no such regularity would be possible, on account
of their many different angles with the stem.]
The bark of the Beech is beautifully smooth. The extreme straightness of
the trunk and limbs is very striking, and may be compared to the crooked
limbs of the Horsechestnut, where the branch is continually interrupted by
the flower-cluster. In the Beech the flowers are axillary.
QUESTIONS ON THE BEECH.
How are the scales of the Beech bud arranged?
How many leaves are there in the bud?
How does the arrangement of the scales and leaves in the bud differ from
that of the Horsechestnut?
How are the leaves folded in the bud?
What is the arrangement of the leaves on the stem?
How does this differ from Horsechestnut and Lilac?
How old is your branch?
How old is each twig?
What years were the best for growth?
How does the growth of the branches differ from that of Horsechestnut?
From Lilac?
Explain these differences with reference to the growth and arrangement of
the buds?
In what direction do the twigs grow?
How does this affect the appearance of the tree?
Compare the amount of spray of the Beech and Horsechestnut and explain the
reason of the difference.
These questions are only intended for review, they are never to be used
for the first study of the specimen.
AMERICAN ELM (_Ulmus Americana_).
The buds are covered with brown scales, which are hairy on the edges. The
flower-buds are larger than the leaf-buds and are in the axils of the
lower leaves of the preceding year. Each leaf in the bud is enclosed by
a pair of scales. They are so small that the pupils, unused to delicate
work, will hardly discover them. Under a glass they can be seen to be
ovate, folded on the midrib with the inner face within (_conduplicate_),
and with an ovate scale joined to the base of the leaf on either side. The
scales thus show themselves to be modified stipules. The venation of the
leaves is very plain. The scales are much larger than the leaves. The
flower-buds contain a cluster of flowers, on slender green pedicels. The
calyx is bell-shaped, unequal, and lobed. The stamens and pistil can
be seen. The flower-clusters do not seem to leave any mark which is
distinguishable from the leaf-scar.
[Illustration: FIG. 16.--American Elm. 1. Branch in winter state: _a_,
leaf-scars; _b_, bud-scars; _d_, leaf-buds; _e_, flower-buds. 2. Branch,
with staminate flower-buds expanding. 3. Same, more advanced. 4. Branch,
with pistillate flowers, the leaf-bud also expanding.]
The leaf-scars are small and extend about half around the stem. The
arrangement is alternate on the one-half plan. There are three dots on the
scar.
The rings are quite plain. The tree can be used to make tables of growth,
like those of the Beech.
The buds will probably be too small for examination by the pupils, at
present, but their position and development can be studied, and are very
instructive. As the leaf-buds are all on the ends of the branchlets, the
twigs and branches will be just below the bud-rings, and then there will
be a space where no twigs nor branches will be found, till the next set
of rings is reached. This gives the branches more room to develop
symmetrically. The terminal buds do not develop in the Elm, in old trees,
the bud axillary to the last leaf of the season taking its place, and most
of the other axillary buds growing also. This makes the tree break out
into very fine spray. A tree like the Elm, where the trunk becomes lost in
the branches, is called _deliquescent_; when the trunk is continued to the
top of the tree, as in the Spruce, it is _excurrent_.
The small, feathery twigs and branches that are often seen on the trunks
and great limbs of the elm grow from buds which are produced anywhere on
the surface of the wood. Such buds are called _adventitious_ buds. They
often spring from a tree when it is wounded.
"The American elm is, in most parts of the state, the most magnificent
tree to be seen. From a root, which, in old trees, spreads much above
the surface of the ground, the trunk rises to a considerable height in a
single stem. Here it usually divides into two or three principal branches,
which go off by a gradual and easy curve. Theses stretch upwards and
outwards with an airy sweep, become horizontal, the extreme half of the
limb, pendent, forming a light and regular arch. This graceful curvature,
and absence of all abruptness, in the primary limbs and forks, and all the
subsequent divisions, are entirely characteristic of the tree, and enable
an observer to distinguish it in the winter and even by night, when
standing in relief against the sky, as far as it can be distinctly
seen."[1]
[Footnote 1: A Report on the Trees and Shrubs growing naturally in the
Forests of Massachusetts. By Geo. B. Emerson, Boston, Little, Brown and
Co., 1875.
This book will be found very useful, containing careful descriptions of
many trees and shrubs, and interesting facts about them.]
QUESTIONS ON THE AMERICAN ELM.
How do the flower-buds differ from the leaf-buds in position and
appearance?
What is the arrangement of the leaves?
What other tree that you have studied has this arrangement?
How old is your branch?
Where would you look to see if the flower-cluster had left any mark?
Why is it that several twigs grow near each other, and that then comes a
space without any branches?
What buds develop most frequently?
How does this affect the appearance of the tree?
What is a tree called when the trunk is lost in the branches?
BALM OF GILEAD (_Populus balsamifera, var. candicans_).
The buds are pointed: the terminal slightly angled, the axillary flattened
against the stem.[1] Some of the axillary buds contain leaves and some
flowers; the appearance of the leaf-buds and flower-buds being the same.
The scales of the bud are modified stipules. The terminal buds have about
three pairs of the outer scales brown and leathery. The inner scales, as
well as the leaves, are coated with resinous matter, which has a strong
odor and a nauseous taste. The smaller outer scales have no corresponding
leaf, and apparently are modified stipules of the leaves of the preceding
year, but the larger ones have a leaf to each pair of scales. The outer
and inner leaves are small, the middle ones larger. Comparing the branch,
it will be seen that these leaves make the largest growth of internode.
The leaves are rolled towards the midrib on the upper face (_involute_).
There are about ten which are easily seen and counted, the inner ones
being very small, with minute scales. The axillary buds have a short
thick scale on the outer part of the bud, then about three pairs of large
scales, each succeeding one enwrapping those within, the outer one brown
and leathery. The scales of the flower-buds are somewhat gummy, but not
nearly so much so as those of the leaf-buds. Within is the catkin. Each
pistil, or stamen (they are on separate trees, _dioecious_) is in a little
cup and covered by a scale, which is cut and fringed.
[Footnote 1: These buds cannot be satisfactorily examined in cross
section, on account of the resin. The scales must be removed one by one,
with a knife, with a complete disregard of the effect upon the hands.]
The leaf-scars are somewhat three-lobed on the young parts, with three
dots, indicating the fibro-vascular bundles, which ran up into the leaf.
The scars are swollen, making the young branches exceedingly rough. In
the older parts the scars become less noticeable. Strong young shoots,
especially those which come up from the root, are strongly angled,
with three ridges running up into each leaf-scar, making them almost
club-shaped. There are often from twenty to thirty leaves in one year's
growth, in such shoots, and all the leaves are not rudimentary in the bud.
The growth in this case is said to be _indefinite_. Usually in trees with
scaly buds the plan of the whole year's growth is laid down in the bud,
and the term _definite_ is applied. Branches, like the Rose, that go on
growing all summer grow indefinitely.
The bud-scale scar is quite different from the other trees which we have
examined. It is not composed of definite rings, but of leaf-scars with
long ridges running from each side of them, showing the scales to be
modified stipules. The leaf-scars have become somewhat separated by the
growth of the internodes. In the Beech, there are eight, or more, pairs of
scales with no leaves, so that the internodes do not develop, and a ring
is left on the branch.
The flower-cluster leaves a concave, semicircular scar, in the leaf-axil.
[Illustration: FIG. 17.--Balm-of-Gilead. 1. Branch in winter state: _a_,
leaf-scar; _b_, bud-scar. 2. Branch, with leaf-buds expanded. 3. Branch,
with catkin appearing from the bud.]
The terminal buds are the strongest and not very many axillary buds
develop, so that the tree has not fine spray.
The leaf-arrangement is alternate, on the 2/5 plan. Phyllotaxy is not yet
to be taken up, but the pupils should be shown the different angles of the
branching of the twigs, and told to compare them with Beech and Elm.
QUESTIONS ON THE BALM OF GILEAD.
In which buds are the flower-clusters?
Are there flowers and leaves in the same buds?
What are the scales of the bud?
How are the leaves folded in the bud?
How do the axillary and terminal buds differ?
What are the dots on the leaf-scars?
Why is there no distinct band of rings as in Beech?
How old is your branch?
Where do you look for flower-cluster scars?
Which buds are the strongest?
How does this affect the appearance of the tree?
What makes the ends of the branches so rough?
Compare the arrangement of the twigs and branches with Beech and Elm, with
Horsechestnut and Lilac.
TULIP-TREE (_Liriodendron Tulipifera_).
The buds are small, flat, and rounded at the apex. They are sheathed by
scales, each leaf being covered by a pair, whose edges cohere. The outer
pair are brown and are the stipules of the last leaf of the preceding
year. The leaves are conduplicate, as in Magnolia, and have the blade bent
inwards on the petiole (_inflexed_). Their shape is very clearly to be
seen, and no bud is more interesting in the closeness of its packing.
Axillary buds are often found within. The flowers grow high upon the trees
and towards the ends of the branches.
The leaf-scars are round with many dots. The scar of the stipules is a
continuous line around the stem, as in Magnolia.
CHERRY _(Prunus Cerasus_).
The leaf-buds are terminal, or in the axils of the upper leaves of the
preceding year; the flower buds are axillary. There is but one bud in each
axil, and usually two or three flowers in each bud, but the leaves on
the twigs are crowded and the flowers therefore appear in clusters. The
blossom-buds are larger and more rounded than the leaf-buds.
The buds of the tree develop very easily in the house, and as they are
so small they can be better studied in watching them come out, than by
attempting to dissect them, unless the scholars are sufficiently advanced
to use the microscope easily. It is always bad for a pupil to attempt to
describe what he sees but imperfectly. He will be sure to jump at any
conclusions which he thinks ought to be correct.
The leaf-scars are semicircular, small and swollen.
The bud-rings are plain. The twigs make a very small growth in a season,
so that the leaf-scars and rings make them exceedingly rough.
The flower-cluster scars are small circles, with a dot in the centre, in
the leaf-axils. The flowers come before the leaves.
The leaf-arrangement is alternate on the 2/5 plan. The pupils may compare
the branching with that of their other specimens.
RED MAPLE (_Acer rubrum_).
This is a good specimen for the study of accessory buds. There is usually
a bud in the axil of each lower scale of the axillary buds, making three
side by side. We have already noticed this as occurring sometimes in
Lilac. It is habitually the case with the Red Maple. The middle bud, which
is smaller and develops later, is a leaf-bud. The others are flower-buds.
The leaf-scars are small, with three dots on each scar. The rings are very
plain. The flower-cluster leaves a round scar in the leaf-axil, as in
Cherry.
The leaves are opposite and the tree branches freely. The twigs seem to
be found just below the bud-rings, as the upper leaf-buds usually develop
best and the lower buds are single, containing flowers only.
NORWAY SPRUCE (_Picea excelsa_).
The buds are terminal, and axillary, from the axils of the leaves of the
preceding year, usually from those at the ends of the branchlets. They are
covered with brown scales and contain many leaves.
[Illustration: FIG. 18.--Branch of Cherry in winter state: _a_, leaf-scar;
_b_, bud-scar; _c_, flower-scar.]
[Illustration: FIG. 19.--Branch of Red Maple in winter state (reduced). 2.
Flower-buds]
The leaves are needle-shaped and short.[1] They are arranged densely on
the branches, alternately on the 8/21 plan (see section on phyllotaxy).
When they drop off they leave a hard, blunt projection which makes the
stem very rough. As the terminal bud always develops unless injured, the
tree is excurrent, forming a straight trunk, throwing out branches on
every side. The axillary buds develop near the ends of the branchlets,
forming apparent whorls of branches around the trunk. In the smaller
branches, as the tree grows older, the tendency is for only two buds to
develop nearly opposite each other, forming a symmetrical branch.
[Footnote 1: The pupils should observe how much more crowded the leaves
are than in the other trees they have studied. The leaves being smaller,
it is necessary to have more of them. Large-leaved trees have longer
internodes than those with small leaves.]
The bud-scales are persistent on the branches and the growth from year to
year can be traced a long way back.
The cones hang on the ends of the upper branches. They are much larger
than in our native species of Black and White Spruce.
The Evergreens are a very interesting study and an excellent exercise in
morphology for the older scholars.
2. _Vernation_. This term signifies the disposition of leaves in the bud,
either in respect to the way in which each leaf is folded, or to the
manner in which the leaves are arranged with reference to each other.
The pupils have described the folding of the leaves in some of their
specimens.
In the Beech, the leaf is _plicate_, or plaited on the veins. In the Elm,
Magnolia, and Tulip-tree, it is _conduplicate_, that is, folded on
the midrib with the inner face within. In the Tulip-tree, it is also
_inflexed_, the blade bent forwards on the petiole. In the Balm of Gilead,
the leaf is _involute_, rolled towards the midrib on the upper face.
Other kinds of vernation are _revolute_, the opposite of involute, where
the leaf is rolled backwards towards the midrib; _circinate_, rolled from
the apex downwards, as we see in ferns; and _corrugate_, when the leaf is
crumpled in the bud.
[Illustration: FIG. 20.--Branch of Norway Spruce.]
In all the trees we have studied, the leaves simply succeed each other,
each leaf, or pair of leaves, overlapping the next in order. The names of
the overlapping of the leaves among themselves, _imbricated, convolute,
etc_., will not be treated here, as they are not needed. They will come
under _æstivation_, the term used to describe the overlapping of the
modified leaves, which make up the flower.[1]
[Footnote 1: Reader in Botany. VIII. Young and Old Leaves.]
3. _Phyllotaxy_. The subject of leaf-arrangement is an extremely difficult
one, and it is best, even with the older pupils, to touch it lightly. The
point to be especially brought out is the disposition of the leaves so
that each can get the benefit of the light. This can be seen in any plant
and there are many ways in which the desired result is brought about. The
chief way is the distribution of the leaves about the stem, and this is
well studied from the leaf-scars.
The scholars should keep the branches they have studied. It is well to
have them marked with the respective names, that the teacher may examine
and return them without fear of mistakes.
In the various branches that the pupils have studied, they have seen that
the arrangement of the leaves differs greatly. The arrangement of leaves
is usually classed under three modes: the _alternate_, the _opposite_,
and the _whorled_; but the opposite is the simplest form of the whorled
arrangement, the leaves being in circles of two. In this arrangement, the
leaves of each whorl stand over the spaces of the whorl just below. The
pupils have observed and noted this in Horsechestnut and Lilac. In these
there are four vertical rows or ranks of leaves. In whorls of three leaves
there would be six ranks, in whorls of four, eight, and so on.
When the leaves are alternate, or single at each node of the stem, they
are arranged in many different ways. Ask the pupils to look at all the
branches with alternate leaves that they have studied, and determine in
each case what leaves stand directly over each other. That is, beginning
with any leaf, count the number of leaves passed on the stem, till one is
reached that stands directly over the first.[1] In the Beech and the Elm
the leaves are on opposite sides of the stem, so that the third stands
directly over the first. This makes two vertical ranks, or rows, of
leaves, dividing the circle into halves. It is, therefore, called the
1/2 arrangement. Another way of expressing it is to say that the angular
divergence between the leaves is 180°, or one-half the circumference.
[Footnote 1: The pupils must be careful not to pass the bud-rings when
they are counting the leaves.]
The 1/3 arrangement, with the leaves in three vertical ranks, is not very
common. It may be seen in Sedges, in the Orange-tree, and in Black Alder
_(Ilex verticillata)_. In this arrangement, there are three ranks of
leaves, and each leaf diverges from the next at an angle of 120°, or
one-third of the circumference.
By far the commonest arrangement is with the leaves in five vertical
ranks. The Cherry, the Poplar, the Larch, the Oak, and many other trees
exhibit this. In this arrangement there are five leaves necessary to
complete the circle. We might expect, then, that each leaf would occupy
one-fifth of the circle. This would be the case were it not for the fact
that we have to pass twice around the stem in counting them, so that each
leaf has twice as much room, or two-fifths of the circle, to itself. This
is, therefore, the 2/5 arrangement. This can be shown by winding a thread
around the stem, passing it over each leaf-scar. In the Beech we make one
turn of the stem before reaching the third leaf which stands over the
first. In the Apple the thread will wind twice about the stem, before
coming to the sixth leaf, which is over the first.
Another arrangement, not very common, is found in the Magnolia, the Holly,
and the radical leaves of the common Plantain and Tobacco. The thread
makes three turns of the stem before reaching the eighth leaf which stands
over the first. This is the 3/8 arrangement. It is well seen in the
Marguerite, a greenhouse plant which is very easily grown in the house.
Look now at these fractions, 1/2, 1/3, 2/5, and 3/8. The numerator of
the third is the sum of the numerators of the first and second, its
denominator, the sum of the two denominators. The same is true of the
fourth fraction and the two immediately preceding it. Continuing the
series, we get the fractions 5/13, 8/21, 13/34. These arrangements can
be found in nature in cones, the scales of which are modified leaves and
follow the laws of leaf-arrangement.[1]
[Footnote 1: See the uses and origin of the arrangement of leaves in
plants. By Chauncey Wright. Memoirs Amer. Acad., IX, p. 389. This essay
is an abstruse mathematical treatise on the theory of phyllotaxy. The
fractions are treated as successive approximations to a theoretical angle,
which represents the best possible exposure to air and light.
Modern authors, however, do not generally accept this mathematical view of
leaf-arrangement.]
[1]"It is to be noted that the distichous or 1/2 variety gives the maximum
divergence, namely 180°, and that the tristichous, or 1/3, gives the
least, or 120°; that the pentastichous, or 2/5, is nearly the mean between
the first two; that of the 3/8, nearly the mean between the two preceding,
etc. The disadvantage of the two-ranked arrangement is that the leaves are
soon superposed and so overshadow each other. This is commonly obviated by
the length of the internodes, which is apt to be much greater in this
than in the more complex arrangements, therefore placing them vertically
further apart; or else, as in Elms, Beeches, and the like, the branchlets
take a horizontal position and the petioles a quarter twist, which gives
full exposure of the upper face of all the leaves to the light. The 1/3
and 2/5, with diminished divergence, increase the number of ranks; the 3/8
and all beyond, with mean divergence of successive leaves, effect a more
thorough distribution, but with less and less angular distance between the
vertical ranks."
[Footnote 1: Gray's Structural Botany, Chap, iv, p. 126.]
For directions for finding the arrangement of cones, see Gray's Structural
Botany, Chap. IV, Sect. 1.
The subject appears easy when stated in a text-book, but, practically, it
is often exceedingly difficult to determine the arrangement. Stems often
twist so as to alter entirely the apparent disposition of the leaves. The
general principle, however, that the leaves are disposed so as to get the
best exposure to air and light is clear. This cannot be shown by the study
of the naked branches merely, because these do not show the beautiful
result of the distribution.[1] Many house plants can be found, which will
afford excellent illustrations (Fig. 21). The Marguerite and Tobacco, both
easily grown in the house, are on the 3/8 plan. The latter shows the eight
ranks most plainly in the rosette of its lower leaves. The distribution is
often brought about by differences in the lengths of the petioles, as in
a Horsechestnut branch (Fig. 22) where the lower, larger leaves stand
out further from the branch than the upper ones; or by a twist in the
petioles, so that the upper faces of the leaves are turned up to the
light, as in Beech (Fig. 23). If it is springtime when the lessons are
given, endless adaptations can be found.
[Footnote 1: Reader in Botany. IX. Leaf-Arrangement.]
[Illustration: FIG. 21. Branch of Geranium, viewed from above.]
[Illustration: FIG. 22.]
[Illustration: FIG. 23.]
_Gray's First Lessons_. Sect. IV. VII, §4. _How Plants Grow_. Chap. I,
51-62; I, 153.
V.
STEMS.
The stem, as the scholars have already learned, is the axis of the plant.
The leaves are produced at certain definite points called nodes, and the
portions of stem between these points are internodes. The internode,
node, and leaf make a single plant-part, and the plant is made up of a
succession of such parts.
The stem, as well as the root and leaves, may bear plant-hairs. The
accepted theory of plant structure assumes that these four parts, root,
stem, leaves, and plant-hairs, are the only members of a flowering plant,
and that all other forms, as flowers, tendrils, etc., are modified from
these. While this idea is at the foundation of all our teaching, causing
us to lead the pupil to recognize as modified leaves the cotyledons of a
seedling and the scales of a bud, it is difficult to state it directly
so as to be understood, except by mature minds. I have been frequently
surprised at the failure of even bright and advanced pupils to grasp this
idea, and believe it is better to let them first imbibe it unconsciously
in their study. Whenever their minds are ready for it, it will be readily
understood. The chief difficulty is that they imagine that there is a
direct metamorphosis of a leaf to a petal or a stamen.
Briefly, the theory is this: the beginnings of leaf, petal, tendril, etc.,
are the same. At an early stage of their growth it is impossible to tell
what they are to become. They develop into the organ needed for the
particular work required of them to do. The organ, that under other
circumstances might develop into a leaf, is capable of developing into a
petal, a stamen, or a pistil, according to the requirements of the plant,
but no actual metamorphosis takes place. Sometimes, instead of developing
into the form we should normally find, the organ develops into another
form, as when a petal stands in the place of a stamen, or the pistil
reverts to a leafy branch. This will be more fully treated under flowers.
The study of the different forms in which an organ may appear is the study
of _morphology_.
1. _Forms of Stems_.--Stems may grow in many ways. Let the pupils compare
the habits of growth of the seedlings they have studied. The Sunflower and
Corn are _erect_. This is the most usual habit, as with our common trees.
The Morning Glory is _twining_, the stem itself twists about a support.
The Bean, Pea and Nasturtium are _climbing_. The stems are weak, and
are held up, in the first two by tendrils, in the last by the twining
leaf-stalks. The English Ivy, as we have seen, is also climbing, by means
of its aërial roots. The Red Clover is _ascending_, the branches rising
obliquely from the base. Some kinds of Clover, as the White Clover, are
_creeping_, that is, with prostrate branches rooting at the nodes and
forming new plants. Such rooting branches are called _stolons_, or when
the stem runs underground, _suckers_. The gardener imitates them in
the process called layering, that is, bending down an erect branch and
covering it with soil, causing it to strike root. When the connecting stem
is cut, a new plant is formed. Long and leafless stolons, like those of
the Strawberry are called _runners_. Stems creep below the ground as well
as above. Probably the pupil will think of some examples. The pretty
little Gold Thread is so named from the yellow running stems, which grow
beneath the ground and send up shoots, or suckers, which make new plants.
Many grasses propagate themselves in this way. Such stems are called
_rootstocks_. "That these are really stems, and not roots, is evident
from the way in which they grow; from their consisting of a succession of
joints; and from the leaves which they bear on each node, in the form
of small scales, just like the lowest ones on the upright stem next the
ground. They also produce buds in the axils of these scales, showing the
scales to be leaves; whereas real roots bear neither leaves nor axillary
buds."[1] Rootstocks are often stored with nourishment. We have already
taken up this subject in the potato, but it is well to repeat the
distinction between stems and roots. A thick, short rootstock provided
with buds, like the potato, is called a _tuber_. Compare again the corm of
Crocus and the bulb of Onion to find the stem in each. In the former, it
makes the bulk of the whole; in the latter, it is a mere plate holding the
fleshy bases of the leaves.
[Footnote 1: Gray's First Lessons, revised edition, 1887, page 42.]
2. _Movements of Stems.--_Let a glass thread, no larger than a coarse
hair, be affixed by means of some quickly drying varnish to the tip of the
laterally inclined stem of one of the young Morning-Glory plants in the
schoolroom. Stand a piece of cardboard beside the pot, at right angles to
the stem, so that the end of the glass will be near the surface of the
card. Make a dot upon the card opposite the tip of the filament, taking
care not to disturb the position of either. In a few minutes observe that
the filament is no longer opposite the dot. Mark its position anew, and
continue thus until a circle is completed on the cardboard. This is a
rough way of conducting the experiment. Darwin's method will be found in
the footnote.[1]
[Footnote 1: "Plants growing in pots were protected wholly from the light,
or had light admitted from above or on one side as the case might require,
and were covered above by a large horizontal sheet of glass, and with
another vertical sheet on one side. A glass filament, not thicker than a
horsehair, and from a quarter to three-quarters of an inch in length,
was affixed to the part to be observed by means of shellac dissolved in
alcohol. The solution was allowed to evaporate until it became so thick
that it set hard in two or three seconds, and it never injured the
tissues, even the tips of tender radicles, to which it was applied. To the
end of the glass filament an excessively minute bead of black sealing-wax
was cemented, below or behind which a bit of card with a black dot was
fixed to a stick driven into the ground.... The bead and the dot on the
card were viewed through the horizontal or vertical glass-plate (according
to the position of the object) and when one exactly covered the other, a
dot was made on the glass plate with a sharply pointed stick dipped in
thick India ink. Other dots were made at short intervals of time and these
were afterwards joined by straight lines. The figures thus traced were
therefore angular, but if dots had been made every one or two minutes, the
lines would have been more curvilinear."--The Power of Movement in Plants,
p. 6.]
The use of the glass filament is simply to increase the size of the circle
described, and thus make visible the movements of the stem. All young
parts of stems are continually moving in circles or ellipses. "To learn
how the sweeps are made, one has only to mark a line of dots along the
upper side of the outstretched revolving end of such a stem, and to note
that when it has moved round a quarter of a circle, these dots will be on
one side; when half round, the dots occupy the lower side; and when the
revolution is completed, they are again on the upper side. That is, the
stem revolves by bowing itself over to one side,--is either pulled over or
pushed over, or both, by some internal force, which acts in turn all round
the stem in the direction in which it sweeps; and so the stem makes its
circuits without twisting."[1]
[Footnote 1: How Plants Behave. By Asa Gray. Ivison, Blakeman, Taylor &
Co., New York, 1872. Page 13.]
The nature of the movement is thus a successive nodding to all the points
of the compass, whence it is called by Darwin _circumnutation_. The
movement belongs to all young growing parts of plants. The great sweeps of
a twining stem, like that of the Morning-Glory, are only an increase in
the size of the circle or ellipse described.[1]
[Footnote 1: "In the course of the present volume it will be shown
that apparently every growing part of every plant is continually
circumnutating, though often on a small scale. Even the stems of seedlings
before they have broken through the ground, as well as their buried
radicles, circumnutate, as far as the pressure of the surrounding earth
permits. In this universally present movement we have the basis or
groundwork for the acquirement, according to the requirements of the
plant, of the most diversified movements. Thus the great sweeps made by
the stems of the twining plants, and by the tendrils of other climbers,
result from a mere increase in the amplitude of the ordinary movement of
circumnutation."--The Power of Movement in Plants, p. 3.]
When a young stem of a Morning-Glory, thus revolving, comes in contact
with a support, it will twist around it, unless the surface is too smooth
to present any resistance to the movement of the plant. Try to make
it twine up a glass rod. It will slip up the rod and fall off. The
Morning-Glory and most twiners move around from left to right like the
hands of a clock, but a few turn from right to left.
While this subject is under consideration, the tendrils of the Pea and
Bean and the twining petioles of the Nasturtium will be interesting for
comparison. The movements can be made visible by the same method as was
used for the stem of the Morning-Glory. Tendrils and leaf petioles are
often sensitive to the touch. If a young leaf stalk of Clematis be rubbed
for a few moments, especially on the under side, it will be found in a day
or two to be turned inward, and the tendrils of the Cucumber vine will
coil in a few minutes after being thus irritated.[1] The movements of
tendrils are charmingly described in the chapter entitled "How Plants
Climb," in the little treatise by Dr. Gray, already mentioned.
[Footnote 1: Reader in Botany. X. Climbing Plants.]
The so-called "sleep of plants" is another similar movement. The Oxalis is
a good example. The leaves droop and close together at night, protecting
them from being chilled by too great radiation.
The cause of these movements is believed to lie in changes of tension
preceding growth in the tissues of the stem.[1] Every stem is in a state
of constant tension. Naudin has thus expressed it, "the interior of every
stem is too large for its Jacket."[2] If a leaf-stalk of Nasturtium be
slit vertically for an inch or two, the two halves will spring back
abruptly. This is because the outer tissues of the stem are stretched,
and spring back like india-rubber when released. If two stalks twining
in opposite directions be slit as above described, the side of the stem
towards which each stalk is bent will spring back more than the other,
showing the tension to be greater on that side. A familiar illustration of
this tension will be found in the Dandelion curls of our childhood.
[Footnote 1: See Physiological Botany. By Geo. L. Goodale. Ivison & Co.,
New York, 1885. Page 406.]
[Footnote 2: The following experiment exhibits the phenomenon of tension
very strikingly. "From a long and thrifty young internode of grapevine
cut a piece that shall measure exactly one hundred units, for instance,
millimeters. From this section, which measures exactly one hundred
millimeters, carefully separate the epidermal structures in strips, and
place the strips at once under an inverted glass to prevent drying;
next, separate the pith in a single unbroken piece wholly freed from the
ligneous tissue. Finally, remeasure the isolated portions, and compare
with the original measure of the internode. There will be found an
appreciable shortening of the epidermal tissues and a marked increase in
length of the pith."--Physiological Botany, p. 391.]
The movements of the Sensitive Plant are always very interesting to
pupils, and it is said not to be difficult to raise the plants in the
schoolroom. The whole subject, indeed, is one of the most fascinating
that can be found, and its literature is available, both for students and
teachers. Darwin's essay on "Climbing Plants," and his later work on the
"Power of Movement in Plants," Dr. Gray's "How Plants Behave," and the
chapter on "Movements" in the "Physiological Botany," will offer a wide
field for study and experiment.
3. _Structure of Stems_.--Let the pupils collect a series of branches of
some common tree or shrub, from the youngest twig up to as large a branch
as they can cut, and describe them. Poplar, Elm, Oak, Lilac, etc., will be
found excellent for the purpose.
While discussing these descriptions, a brief explanation of
plant-structure may be given. In treating this subject, the teacher must
govern himself by the needs of his class, and the means at his command.
Explanations requiring the use of a compound microscope do not enter
necessarily into these lessons. The object aimed at is to teach the pupils
about the things which they can see and handle for themselves. Looking at
sections that others have prepared is like looking at pictures; and, while
useful in opening their eyes and minds to the wonders hidden from our
unassisted sight, fails to give the real benefit of scientific training.
Plants are built up of cells. The delicate-walled spherical, or polygonal,
cells which make up the bulk of an herbaceous stem, constitute cellular
tissue (_parenchyma_). This was well seen in the stem of the cutting of
Bean in which the roots had begun to form.[1] The strengthening fabric
in almost all flowering plants is made up of woody bundles, or woody
tissue.[2] The wood-cells are cells which are elongated and with thickened
walls. There are many kinds of them. Those where the walls are very thick
and the cavity within extremely small are _fibres_. A kind of cell, not
strictly woody, is where many cells form long vessels by the breaking away
of the connecting walls. These are _ducts_. These two kinds of cells
are generally associated together in woody bundles, called therefore
fibro-vascular bundles. We have already spoken of them as making the dots
on the leaf-scars, and forming the strengthening fabric of the leaves.[3]
[Footnote 1: See page 46.]
[Footnote 2: If elements of the same kind are untied, they constitute a
tissue to which is given the name of those elements; thus parenchyma cells
form parenchyma tissue or simply parenchyma; cork-cells form cork, etc. A
tissue can therefore be defined as a fabric of united cells which have had
a common origin and obeyed a common law of growth.--Physiological Botany.
p. 102.]
[Footnote 3: See page 58.]
We will now examine our series of branches. The youngest twigs, in spring
or early summer, are covered with a delicate, nearly colorless skin.
Beneath this is a layer of bark, usually green, which gives the color to
the stem, an inner layer of bark, the wood and the pith. The pith is soft,
spongy and somewhat sappy. There is also sap between the bark and the
wood. An older twig has changed its color. There is a layer of brown bark,
which has replaced the colorless skin. In a twig a year old the wood is
thicker and the pith is dryer. Comparing sections of older branches with
these twigs, we find that the pith has shrunk and become quite dry, and
that the wood is in rings. It is not practicable for the pupils to
compare the number of these rings with the bud-rings, and so find out for
themselves that the age of the branch can be determined from the wood, for
in young stems the successive layers are not generally distinct. But, in
all the specimens, the sap is found just between the wood and the bark,
and here, where the supply of food is, is where the growth is taking
place. Each year new wood and new bark are formed in this _cambium-layer_,
as it is called, new wood on its inner, new bark on its outer face. Trees
which thus form a new ring of wood every year are called _exogenous_, or
outside-growing.
Ask the pupils to separate the bark into its three layers and to try
the strength of each. The two outer will easily break, but the inner is
generally tough and flexible. It is this inner bark, which makes the
Poplar and Willow branches so hard to break. These strong, woody fibres
of the inner bark give us many of our textile fabrics. Flax and Hemp come
from the inner bark of their respective plants (_Linum usitatissimum_ and
_Cannabis sativa_), and Russia matting is made from the bark of the Linden
(_Tilia Americana_).
We have found, in comparing the bark of specimens of branches of various
ages, that, in the youngest stems, the whole is covered with a skin, or
_epidermis_, which is soon replaced by a brown outer layer of bark, called
the _corky layer_; the latter gives the distinctive color to the tree.
While this grows, it increases by a living layer of cork-cambium on its
inner face, but it usually dies after a few years. In some trees it goes
on growing for many years. It forms the layers of bark in the Paper Birch
and the cork of commerce is taken from the Cork Oak of Spain. The green
bark is of cellular tissue, with some green coloring matter like that of
the leaves; it is at first the outer layer, but soon becomes covered with
cork. It does not usually grow after the first year. Scraping the bark of
an old tree, we find the bark homogeneous. The outer layers have perished
and been cast off. As the tree grows from within, the bark is stretched
and, if not replaced, cracks and falls away piecemeal. So, in most old
trees, the bark consists of successive layers of the inner woody bark.
Stems can be well studied from pieces of wood from the woodpile. The ends
of the log will show the concentric rings. These can be traced as long,
wavy lines in vertical sections of the log, especially if the surface is
smooth. If the pupils can whittle off different planes for themselves,
they will form a good idea of the formation of the wood. In many of
the specimens there will be knots, and the nature of these will be an
interesting subject for questions. If the knot is near the centre of the
log, lead back their thoughts to the time when the tree was as small as
the annular ring on which the centre of the knot lies. Draw a line on this
ring to represent the tree at this period of its growth. What could the
knot have been? It has concentric circles like the tree itself. It was a
branch which decayed, or was cut off. Year after year, new rings of wood
formed themselves round this broken branch, till it was covered from
sight, and every year left it more deeply buried in the trunk.
Extremely interesting material for the study of wood will be found in thin
sections prepared for veneers. Packages of such sections will be of great
use to the teacher.[1] They show well the reason of the formation of a
dividing line between the wood of successive seasons. In a cross section
of Oak or Chestnut the wood is first very open and porous and then close.
This is owing to the presence of ducts in the wood formed in the spring.
In other woods there are no ducts, or they are evenly distributed, but
the transition from the close autumn wood, consisting of smaller and
more closely packed cells, to the wood of looser texture, formed in the
following spring, makes a line that marks the season's growth.
[Footnote 1: Mr. Romeyn B. Hough, of Lowville, N.Y., will supply a package
of such sections for one dollar. The package will consist of several
different woods, in both cross and vertical section and will contain
enough duplicates for an ordinary class.
He also issues a series of books on woods illustrated by actual and neatly
mounted specimens, showing in each case three distinct views of the grain.
The work is issued in parts, each representing twenty-five species, and
selling with text at $5, expressage prepaid; the mounted specimens alone
at 25 cts. per species or twenty-five in neat box for $4. He has also
a line of specimens prepared for the stereopticon and another for the
microscope. They are very useful and sell at 50 cts. per species or
twenty-five for $10.]
Let each of the scholars take one of the sections of Oak and write a
description of its markings. The age is easily determined; the pith rays,
or _medullary rays_, are also plain. These form what is called the silver
grain of the wood. The ducts, also, are clear in the Oak and Chestnut.
There is a difference in color between the outer and inner wood, the older
wood becomes darker and is called the _heart-wood_, the outer is the
_sap-wood_. In Birds-eye Maple, and some other woods, the abortive buds
are seen. They are buried in the wood, and make the disturbance which
produces the ornamental grain. In sections of Pine or Spruce, no ducts
can be found. The wood consists entirely of elongated, thickened cells or
fibres. In some of the trees the pith rays cannot be seen with the naked
eye.
Let the pupils compare the branches which they have described, with a
stalk of Asparagus, Rattan, or Lily. A cross section of one of these shows
dots among the soft tissue. These are ends of the fibro-vascular bundles,
which in these plants are scattered through the cellular tissue instead of
being brought together in a cylinder outside of the pith. In a vertical
section they appear as lines. There are no annular rings.
If possible, let the pupils compare the leaves belonging to these
different types of stems. The parallel-veined leaves of monocotyledons
have stems without distinction of wood, bark and pith; the netted-veined
leaves of dicotyledons have exogenous stems.
Dicotyledons have bark, wood, and pith, and grow by producing a new ring
of wood outside the old. They also increase by the growth of the woody
bundles of the leaves, which mingle with those of the stem.[1] Twist off
the leaf-stalk of any leaf, and trace the bundles into the stem.
[Footnote 1: See note, p. 127, Physiological Botany.]
Monocotyledons have no layer which has the power of producing new wood,
and their growth takes place entirely from the intercalation of new
bundles, which originate at the bases of the leaves. The lower part of a
stem of a Palm, for instance, does not increase in size after it has lost
its crown of leaves. This is carried up gradually. The upper part of the
stem is a cone, having fronds, and below this cone the stem does not
increase in diameter. The word _endogenous_, inside-growing, is not,
therefore, a correct one to describe the growth of most monocotyledons,
for the growth takes place where the leaves originate, near the exterior
of the stem.
_Gray's First Lessons_. Sect. VI. Sect, XVI, §1, 401-13. §3. §6, 465-74.
_How Plants Grow_. Chap. 1, 82, 90-118.
VI.
LEAVES.
We have studied leaves as cotyledons, bud-scales, etc., but when we speak
of _leaves_, we do not think of these adapted forms, but of the green
foliage of the plant.
1. _Forms and Structure_.--Provide the pupils with a number of green
leaves, illustrating simple and compound, pinnate and palmate, sessile and
petioled leaves. They must first decide the question, _What are the parts
of a leaf_? All the specimens have a green _blade_ which, in ordinary
speech, we call the leaf. Some have a stalk, or _petiole_, others are
joined directly to the stem. In some of them, as a rose-leaf, for
instance, there are two appendages at the base of the petiole, called
_stipules_. These three parts are all that any leaf has, and a leaf that
has them all is complete.
Let us examine the blade. Those leaves which have the blade in one
piece are called _simple_; those with the blade in separate pieces are
_compound_. We have already answered the question, _What constitutes a
single leaf_?[1] Let the pupils repeat the experiment of cutting off the
top of a seedling Pea, if it is not already clear in their minds, and find
buds in the leaf-axils of other plants.[2]
[Footnote 1: See page 31.]
[Footnote 2: With one class of children, I had much difficulty in making
them understand the difference between simple and compound leaves. I did
not tell them that the way to tell a single leaf was to look for buds in
the axils, but incautiously drew their attention to the stipules at the
base of a rose leaf as a means of knowing that the whole was one. Soon
after, they had a locust leaf to describe; and, immediately, with the
acuteness that children are apt to develop so inconveniently to their
teacher, they triumphantly refuted my statement that it was one leaf, by
pointing to the stiples. There was no getting over the difficulty; and
although I afterwards explained to them about the position of the buds,
and showed them examples, they clung with true childlike tenacity to their
first impression and always insisted that they could not see why each
leaflet was not a separate leaf.]
An excellent way to show the nature of compound leaves is to mount a
series showing every gradation of cutting, from a simple, serrate leaf to
a compound one (Figs. 24 and 25). A teacher, who would prepare in summer
such illustrations as these, would find them of great use in his winter
lessons. The actual objects make an impression that the cuts in the book
cannot give.
[Illustration: FIG. 24.--Series of palmately-veined leaves.]
[Illustration: FIG. 25.--Series of pinnately-veined leaves.]
Let the pupils compare the distribution of the veins in their specimens.
They have already distinguished parallel-veined from netted-veined leaves,
and learned that this difference is a secondary distinction between
monocotyledons and dicotyledons.[1] The veins in netted-veined leaves are
arranged in two ways. The veins start from either side of a single midrib
(_feather-veined_ or _pinnately-veined_), or they branch from a number of
ribs which all start from the top of the petiole, like the fingers from
the palm of the hand (_palmately-veined_). The compound leaves correspond
to these modes of venation; they are either pinnately or palmately
compound.
[Footnote 1: See page 34.]
These ribs and veins are the woody framework of the leaf, supporting the
soft green pulp. The woody bundles are continuous with those of the stem,
and carry the crude sap, brought from the roots, into the cells of every
part of the leaf, where it is brought into contact with the external
air, and the process of making food (_Assimilation_ 4) is carried on.
"Physiologically, leaves are green expansions borne by the stern,
outspread in the air and light, in which assimilation and the processes
connected with it are carried on."[1]
[Footnote 1: Gray's Structural Botany, p. 85.]
The whole leaf is covered with a delicate skin, or epidermis, continuous
with that of the stem.[1]
[Footnote 1: Reader in Botany. XI. Protection of Leaves from the Attacks
of Animals.]
2. _Descriptions_.--As yet the pupils have had no practice in writing
technical descriptions. This sort of work may be begun when they come to
the study of leaves. In winter a collection of pressed specimens will be
useful. Do not attach importance to the memorizing of terms. Let them be
looked up as they are needed, and they will become fixed by practice. The
pupils may fill out such schedules as the following with any leaves that
are at hand.
SCHEDULE FOR LEAVES.
Arrangement _Alternate_[1]
|Simple or compound. _Simple_
|(arr. and no. of leaflets)
|
|Venation _Netted and
| feather-veined_
|Shape _Oval_
1. BLADE <
| Apex _Acute_
|
| Base _Oblique_
|
|Margin _Slightly wavy_
|
|Surface _Smooth_
2. PETIOLE _Short; hairy_
3. STIPULES _Deciduous_
Remarks. Veins prominent and very straight.
[Footnote 1: The specimen described is a leaf of Copper Beech.]
In describing shapes, etc., the pupils can find the terms in the book as
they need them. It is desirable at first to give leaves that are easily
matched with the terms, keeping those which need compound words, such as
lance-ovate, etc., to come later. The pupils are more interested if they
are allowed to press and keep the specimens they have described. It is not
well to put the pressed leaves in their note books, as it is difficult to
write in the books without spoiling the specimens. It is better to mount
the specimens on white paper, keeping these sheets in brown paper covers.
The pupils can make illustrations for themselves by sorting leaves
according to the shapes, outlines, etc., and mounting them.
3. _Transpiration_.--This term is used to denote the evaporation of water
from a plant. The evaporation takes place principally through breathing
pores, which are scattered all over the surface of leaves and young stems.
The _breathing pores_, or _stomata_, of the leaves, are small openings
in the epidermis through which the air can pass into the interior of the
plant. Each of these openings is called a _stoma_. "They are formed by a
transformation of some of the cells of the epidermis; and consist usually
of a pair of cells (called guardian cells), with an opening between
them, which communicates with an air-chamber within, and thence with the
irregular intercellular spaces which permeate the interior of the leaf.
Through the stomata, when open, free interchange may take place between
the external air and that within the leaf, and thus transpiration be
much facilitated. When closed, this interchange will be interrupted or
impeded."[1]
[Footnote 1: Gray's Structural Botany, page 89. For a description of the
mechanism of the stomata, see Physiological Botany, p. 269.]
In these lessons, however, it is not desirable to enter upon subjects
involving the use of the compound microscope. Dr. Goodale says: "Whether
it is best to try to explain to the pupils the structure of these valves,
or stomata, must be left to each teacher. It would seem advisable to
pass by the subject untouched, unless the teacher has become reasonably
familiar with it by practical microscopical study of leaves. For a teacher
to endeavor to explain the complex structure of the leaf, without having
seen it for himself, is open to the same objection which could be urged
against the attempted explanation of complicated machinery by one who has
never seen it, but has heard about it. What is here said with regard to
stomata applies to all the more recondite matters connected with plant
structure."[1]
[Footnote 1: Concerning a few Common Plants, p. 29.]
There are many simple experiments which can be used to illustrate the
subject.
(1) Pass the stem of a cutting through a cork, fitting tightly into the
neck of a bottle of water. Make the cork perfectly air-tight by coating it
with beeswax or paraffine. The level of the liquid in the bottle will be
lowered by the escape of water through the stem and leaves of the cutting
into the atmosphere.
(2) Cut two shoots of any plant, leave one on the table and place the
other in a glass of water.[1] The first will soon wilt, while the other
will remain fresh. If the latter shoot be a cutting from some plant that
will root in water, such as Ivy, it will not fade at all. Also, leave one
of the plants in the schoolroom unwatered for a day or two, till it begins
to wilt. If the plant be now thoroughly watered, it will recover and the
leaves will resume their normal appearance.
[Footnote 1: Lessons in Elementary Botany, by Daniel Oliver, London.
Macmillan & Co., 1864, pp. 14-15.]
Evaporation is thus constantly taking place from the leaves, and if there
is no moisture to supply the place of what is lost, the cells collapse and
the leaf, as we say, wilts. When water is again supplied the cells swell
and the leaf becomes fresh.
(3) Place two seedlings in water, one with its top, the other with its
roots in the jar. The latter will remain fresh while the first wilts and
dies.
Absorption takes place through the roots. The water absorbed is drawn up
through the woody tissues of the stem (4), and the veins of the leaves
(5), whence it escapes into the air (6).
(4) Plunge a cut branch immediately into a colored solution, such as
aniline red, and after a time make sections in the stem above the liquid
to see what tissues have been stained.[1]
[Footnote 1: The Essentials of Botany, by Charles E. Bessey. New York,
Henry Holt & Co., 1884. Page 74. See also Physiological Botany, pp.
259-260.]
(5) "That water finds its way by preference through the fibro-vascular
bundles even in the more delicate parts, is shown by placing the cut
peduncle of a white tulip, or other large white flower, in a harmless dye,
and then again cutting off its end in order to bring a fresh surface in
contact with the solution,[1] when after a short time the dye will mount
through the flower-stalk and tinge the parts of the perianth according to
the course of the bundles."[2]
[Footnote 1: If the stems of flowers are cut under water they will last a
wonderfully long time. "One of the most interesting characteristics of the
woody tissues in relation to the transfer of water is the immediate change
which the cut surface of a stem undergoes upon exposure to the air,
unfitting it for its full conductive work. De Vries has shown that when a
shoot of a vigorous plant, for instance a Helianthus, is bent down under
water, care being taken not to break it even in the slightest degree,
a clean, sharp cut will give a surface which will retain the power of
absorbing water for a long time; while a similar shoot cut in the open
air, even if the end is instantly plunged under water, will wither much
sooner than the first."--Physiological Botany, p. 263.]
[Footnote 2: Physiological Botany, p. 260.]
(6) Let the leaves of a growing plant rest against the window-pane.
Moisture will be condensed on the cold surface of the glass, wherever the
leaf is in contact with it. This is especially well seen in Nasturtium
(Tropæolum) leaves, which grow directly against a window, and leave the
marks even of their veining on the glass, because the moisture is only
given out from the green tissue, and where the ribs are pressed against
the glass it is left dry.
Sometimes the water is drawn up into the cells of the leaves faster than
it can escape into the atmosphere.[1] This is prettily shown if we place
some of our Nasturtium seedlings under a ward-case. The air in the case is
saturated with moisture, so that evaporation cannot take place, but the
water is, nevertheless, drawn up from the roots and through the branches,
and appears as little drops on the margins of the leaves. That this is
owing to the absorbing power of the roots, may be shown by breaking off
the seedling, and putting the slip in water. No drops now appear on the
leaves, but as soon as the cutting has formed new roots, the drops again
appear.
[Footnote 1: See Lectures on the Physiology of Plants. By Sidney Howard
Vines, Cambridge, England. University Press, 1886. Page 92.]
This constant escape of water from the leaves causes a current to flow
from the roots through the stem into the cells of the leaves. The dilute
mineral solutions absorbed by the roots[1] are thus brought where they
are in contact with the external air, concentrated by the evaporation of
water, and converted in these cells into food materials, such as starch.
The presence of certain mineral matters, as potassium, iron, etc., are
necessary to this assimilating process, but the reason of their necessity
is imperfectly understood, as they do not enter in the products formed.
[Footnote 1: See page 48.]
The amount of water exhaled is often very great. Certain plants are used
for this reason for the drainage of wet and marshy places. The most
important of these is the Eucalyptus tree.[1]
[Footnote 1: Reader in Botany. XII. Transpiration.]
"The amount of water taken from the soil by the trees of a forest and
passed into the air by transpiration is not so large as that accumulated
in the soil by the diminished evaporation under the branches. Hence, there
is an accumulation of water in the shade of forests which is released
slowly by drainage.[1] But if the trees are so scattered as not materially
to reduce evaporation from the ground, the effect of transpiration in
diminishing the moisture of the soil is readily shown. It is noted,
especially in case of large plants having a great extent of exhaling
surface, such, for instance, as the common sunflower. Among the plants
which have been successfully employed in the drainage of marshy soil by
transpiration probably the species of Eucalyptus (notably _E_. _globulus_)
are most efficient."[2]
[Footnote 1: Reader in Botany. XIII. Uses of the Forests.]
[Footnote 2: Physiological Botany, page 283.]
4. _Assimilation_.--It is not easy to find practical experiments on
assimilation. Those which follow are taken from "Physiological Botany" (p.
305).
Fill a five-inch test tube, provided with a foot, with fresh drinking
water. In this place a sprig of one of the following water
plants,--_Elodea Canadensis, Myriophyllum spicatum, M.
verticillatum_, or any leafy _Myriophyllum_ (in fact, any small-
leaved water plant with rather crowded foliage). This sprig should be
prepared as follows: Cut the stem squarely off, four inches or so
from the tip, dry the cut surface quickly with blotting paper, then
cover the end of the stein with a quickly drying varnish, for
instance, asphalt-varnish, and let it dry perfectly, keeping the rest
of the stem, if possible, moist by means of a wet cloth. When the
varnish is dry, puncture it with a needle, and immerse the stem in
the water in the test tube, keeping the varnished larger end
uppermost. If the submerged plant be now exposed to the strong rays
of the sun, bubbles of oxygen gas will begin to pass off at a rapid
and even rate, but not too fast to be easily counted. If the simple
apparatus has begun to give off a regular succession of small
bubbles, the following experiments can be at once conducted:
(1) Substitute for the fresh water some which has been boiled a few
minutes before, and then allowed to completely cool: by the boiling,
all the carbonic acid has been expelled. If the plant is immersed in
this water and exposed to the sun's rays, no bubbles will be evolved;
there is no carbonic acid within reach of the plant for the
assimilative process. But,
(2) If breath from the lungs be passed by means of a slender glass
tube through the water, a part of the carbonic acid exhaled from the
lungs will be dissolved in it, and with this supply of the gas the
plant begins the work of assimilation immediately.
(3) If the light be shut off, the evolution of bubbles will presently
cease, being resumed soon after light again has access to the plant.
(5) Place round the base of the test tube a few fragments of ice, in
order to appreciably lower the temperature of the water. At a certain
point it will be observed that no bubbles are given off, and their
evolution does not begin again until the water becomes warm.
The evolution of bubbles shows that the process of making food is going
on. The materials for this process are carbonic acid gas and water. The
carbonic acid dissolved in the surrounding water is absorbed, the carbon
unites with the elements of water in the cells of the leaves, forming
starch, etc., and most of the oxygen is set free, making the stream of
bubbles. When the water is boiled, the dissolved gas is driven off and
assimilation cannot go on; but as soon as more carbonic acid gas is
supplied, the process again begins. We have seen by these experiments
that sunlight and sufficient heat are necessary to assimilation, and that
carbonic acid gas and water must be present. The presence of the green
coloring matter of the leaves (chlorophyll) is also essential, and some
salts, such as potassium, iron, etc., are needful, though they may not
enter into the compounds formed.
The food products are stored in various parts of the plant for future use,
or are expended immediately in the growth and movements of the plant. In
order that they shall be used for growth, free oxygen is required, and
this is supplied by the respiration of the plant.
Some plants steal their food ready-made. Such a one is the Dodder, which
sends its roots directly into the plant on which it feeds. This is a
_parasite_.[1] It has no need of leaves to carry on the process of making
food. Some parasites with green leaves, like the mistletoe, take the crude
sap from the host-plant and assimilate it in their own green leaves.
Plants that are nourished by decaying matter in the soil are called
_saprophytes_. Indian Pipe and Beech-Drops are examples of this. They need
no green leaves as do plants that are obliged to support themselves.
[Footnote 1: Reader in Botany. XIV. Parasitic Plants.]
Some plants are so made that they can use animal matter for food. This
subject of insectivorous plants is always of great interest to pupils. If
some Sundew (_Drosera_) can be obtained and kept in the schoolroom, it
will supply material for many interesting experiments.[1] That plants
should possess the power of catching insects by specialized movements and
afterwards should digest them by means of a gastric juice like that of
animals, is one of the most interesting of the discoveries that have been
worked out during the last thirty years.[2]
[Footnote 1: See Insectivorous Plants, by Charles Darwin. New York: D.
Appleton and Co., 1875.
How Plants Behave, Chap. III.
A bibliography of the most important works on the subject will be found in
Physiological Botany, page 351, note.]
[Footnote 2: Reader in Botany. XV. Insectivorous Plants.]
5. _Respiration_.--Try the following experiment in germination.
Place some seeds on a sponge under an air-tight glass. Will they grow?
What causes them to mould?
Seeds will not germinate without free access of air. They must have free
oxygen to breathe, as must every living thing. We know that an animal
breathes in oxygen, that the oxygen unites with particles of carbon within
the body and that the resulting carbonic acid gas is exhaled.[1] The same
process goes on in plants, but it was until recently entirely unknown,
because it was completely masked during the daytime by the process of
assimilation, which causes carbonic acid to be inhaled and decomposed, and
oxygen to be exhaled.[2] In the night time the plants are not assimilating
and the process of breathing is not covered up. It has, therefore, long
been known that carbonic acid gas is given off at night. The amount,
however, is so small that it could not injure the air of the room, as
is popularly supposed. Respiration takes place principally through the
stomata of the leaves.[3] We often see plants killed by the wayside dust,
and we all know that on this account it is very difficult to make a hedge
grow well by a dusty road. The dust chokes up the breathing pores of the
leaves, interfering with the action of the plant. It is suffocated.
The oxygen absorbed decomposes starch, or some other food product of the
plant, and carbonic acid gas and water are formed. It is a process of slow
combustion.[4] The energy set free is expended in growth, that is, in the
formation of new cells, and the increase in size of the old ones, and in
the various movements of the plant.
[Footnote 1: See page 13.]
[Footnote 2: This table illustrates the differences between the processes.
ASSIMILATION PROPER. RESPIRATION.
Takes place only in cells Takes place in all active cells.
containing chlorophyll.
Requires light. Can proceed in darkness.
Carbonic acid absorbed, Oxygen absorbed, carbonic
oxygen set free. acid set free.
Carbohydrates formed. Carbohydrates consumed.
Energy of motion becomes Energy of position becomes
energy of position. energy of motion.
The plant gains in dry The plant loses dry weight.
weight.
Physiological Botany, page 356.]
[Transcriber's Note: Two footnote marks [3] and [4] above in original
text, but no footnote text was found in the book]
This process of growth can take place only when living _protoplasm_ is
present in the cells of the plant. The substance we call protoplasm is
an albuminoid, like the white of an egg, and it forms the flesh of both
plants and animals. A living plant can assimilate its own protoplasm, an
animal must take it ready-made from plants. But a plant can assimilate its
food and grow only under the mysterious influence we call life. Life
alone brings forth life, and we are as far as ever from understanding
its nature. Around our little island of knowledge, built up through the
centuries by the labor of countless workers, stretches the infinite ocean
of the unknown.
_Gray's First Lessons_. Sect. VII, XVI, §2, §4, §5, §6, 476-480.
_How Plants Grow_. Chap. I, 119-153, Chap. III, 261-280.
***END OF THE PROJECT GUTENBERG EBOOK OUTLINES OF LESSONS IN BOTANY, PART
I; FROM SEED TO LEAF***
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