Outlines of Lessons in Botany, Part I; from Seed to Leaf · Chapter 6Stems
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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.
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]
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]
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]
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.
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.
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]
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.
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.
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.