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.