Outlines of Lessons in Botany, Part I; From Seed to Leaf - Jane H. Newell
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 (_AEsculus 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.