Outlines of Lessons in Botany, Part I; From Seed to Leaf
Jane H. Newell
Part 2 out of 2
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
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.
The leaves are needle-shaped and short. 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
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 _aestivation_, the term used to describe the overlapping of the
modified leaves, which make up the flower.
[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. 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 deg., 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 deg., 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.
[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
"It is to be noted that the distichous or 1/2 variety gives the maximum
divergence, namely 180 deg., and that the tristichous, or 1/3, gives the
least, or 120 deg.; 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. 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, sec. 4. _How Plants Grow_. Chap. I,
51-62; I, 153.
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
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 aerial 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." 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
[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,
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."
[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.
[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. 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. 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." 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. The strengthening fabric
in almost all flowering plants is made up of woody bundles, or woody
tissue. 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.
[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.
[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
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
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. 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
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. 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, sec. 1, 401-13. sec. 3.
sec. 6, 465-74.
_How Plants Grow_. Chap. 1, 82, 90-118.
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_? 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.
[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
[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. 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
[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."
[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.
[Footnote 1: Reader in Botany. XI. Protection of Leaves from the Attacks
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.
|Simple or compound. _Simple_
|(arr. and no. of leaflets)
|Venation _Netted and
1. BLADE <
| Apex _Acute_
| Base _Oblique_
|Margin _Slightly wavy_
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
[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
[Footnote 1: Concerning a few Common Plants, p. 29.]
There are many simple experiments which can be used to illustrate the
(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. 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
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.
[Footnote 1: The Essentials of Botany, by Charles E. Bessey. New York,
Henry Holt & Co., 1884. Page 74. See also Physiological Botany, pp.
(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, 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."
[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
(Tropaeolum) 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. 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
[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 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.
[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. 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."
[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.
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_. 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. 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.
[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. 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. 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. 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. 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.
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.
Physiological Botany, page 356.]
[Transcriber's Note: Two footnote marks  and  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, sec. 2, sec. 4, sec. 5, sec. 6,
_How Plants Grow_. Chap. I, 119-153, Chap. III, 261-280.
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