We have studied leaves as cotyledons, bud-scales, etc., but when we speak ofleaves, 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 greenbladewhich, in ordinary speech, we call the leaf. Some have a stalk, orpetiole, 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, calledstipules. 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 calledsimple; those with the blade in separate pieces arecompound. We have already answered the question,What constitutes a single leaf?[1] Let the pupils repeat the experiment of cutting off the top of a seedling Pea, if it is not already clear in their minds, and find buds in the leaf-axils of other plants.[2]
An excellent way to show the nature of compound leaves is to mount a series showing every gradation of cutting, from a simple, serrate leaf to a compound one (Figs. 24 and 25). A teacher, who would prepare in summer such illustrations as these, would find them of great use in his winter lessons. The actual objects make an impression that the cuts in the book cannot give.
Series of palmately-veined leaves
[Illustration: FIG. 24.—Series of palmately-veined leaves.]
Series of pinnately-veined leaves
[Illustration: FIG. 25.—Series of pinnately-veined leaves.]
Let the pupils compare the distribution of the veins in their specimens. They have already distinguished parallel-veined from netted-veined leaves, and learned that this difference is a secondary distinction between monocotyledons and dicotyledons.[1] The veins in netted-veined leaves are arranged in two ways. The veins start from either side of a single midrib (feather-veinedorpinnately-veined), or they branch from a number of ribs which all start from the top of the petiole, like the fingers from the palm of the hand (palmately-veined). The compound leaves correspond to these modes of venation; they are either pinnately or palmately compound.
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 (Assimilation4) is carried on. "Physiologically, leaves are green expansions borne by the stern, outspread in the air and light, in which assimilation and the processes connected with it are carried on."[1]
The whole leaf is covered with a delicate skin, or epidermis, continuous with that of the stem.[1]
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.
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. Thebreathing pores, orstomata, 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 astoma. "They are formed by a transformation of some of the cells of the epidermis; and consist usually of a pair of cells (called guardian cells), with an opening between them, which communicates with an air-chamber within, and thence with the irregular intercellular spaces which permeate the interior of the leaf. Through the stomata, when open, free interchange may take place between the external air and that within the leaf, and thus transpiration be much facilitated. When closed, this interchange will be interrupted or impeded."[1]
In these lessons, however, it is not desirable to enter upon subjects involving the use of the compound microscope. Dr. Goodale says: "Whether it is best to try to explain to the pupils the structure of these valves, or stomata, must be left to each teacher. It would seem advisable to pass by the subject untouched, unless the teacher has become reasonably familiar with it by practical microscopical study of leaves. For a teacher to endeavor to explain the complex structure of the leaf, without having seen it for himself, is open to the same objection which could be urged against the attempted explanation of complicated machinery by one who has never seen it, but has heard about it. What is here said with regard to stomata applies to all the more recondite matters connected with plant structure."[1]
There are many simple experiments which can be used to illustrate the subject.
(1) Pass the stem of a cutting through a cork, fitting tightly into the neck of a bottle of water. Make the cork perfectly air-tight by coating it with beeswax or paraffine. The level of the liquid in the bottle will be lowered by the escape of water through the stem and leaves of the cutting into the atmosphere.
(2) Cut two shoots of any plant, leave one on the table and place the other in a glass of water.[1] The first will soon wilt, while the other will remain fresh. If the latter shoot be a cutting from some plant that will root in water, such as Ivy, it will not fade at all. Also, leave one of the plants in the schoolroom unwatered for a day or two, till it begins to wilt. If the plant be now thoroughly watered, it will recover and the leaves will resume their normal appearance.
Evaporation is thus constantly taking place from the leaves, and if there is no moisture to supply the place of what is lost, the cells collapse and the leaf, as we say, wilts. When water is again supplied the cells swell and the leaf becomes fresh.
(3) Place two seedlings in water, one with its top, the other with its roots in the jar. The latter will remain fresh while the first wilts and dies.
Absorption takes place through the roots. The water absorbed is drawn up through the woody tissues of the stem (4), and the veins of the leaves (5), whence it escapes into the air (6).
(4) Plunge a cut branch immediately into a colored solution, such as aniline red, and after a time make sections in the stem above the liquid to see what tissues have been stained.[1]
(5) "That water finds its way by preference through the fibro-vascular bundles even in the more delicate parts, is shown by placing the cut peduncle of a white tulip, or other large white flower, in a harmless dye, and then again cutting off its end in order to bring a fresh surface in contact with the solution,[1] when after a short time the dye will mount through the flower-stalk and tinge the parts of the perianth according to the course of the bundles."[2]
(6) Let the leaves of a growing plant rest against the window-pane. Moisture will be condensed on the cold surface of the glass, wherever the leaf is in contact with it. This is especially well seen in Nasturtium (Tropæolum) leaves, which grow directly against a window, and leave the marks even of their veining on the glass, because the moisture is only given out from the green tissue, and where the ribs are pressed against the glass it is left dry.
Sometimes the water is drawn up into the cells of the leaves faster than it can escape into the atmosphere.[1] This is prettily shown if we place some of our Nasturtium seedlings under a ward-case. The air in the case is saturated with moisture, so that evaporation cannot take place, but the water is, nevertheless, drawn up from the roots and through the branches, and appears as little drops on the margins of the leaves. That this is owing to the absorbing power of the roots, may be shown by breaking off the seedling, and putting the slip in water. No drops now appear on the leaves, but as soon as the cutting has formed new roots, the drops again appear.
This constant escape of water from the leaves causes a current to flow from the roots through the stem into the cells of the leaves. The dilute mineral solutions absorbed by the roots[1] are thus brought where they are in contact with the external air, concentrated by the evaporation of water, and converted in these cells into food materials, such as starch. The presence of certain mineral matters, as potassium, iron, etc., are necessary to this assimilating process, but the reason of their necessity is imperfectly understood, as they do not enter in the products formed.
The amount of water exhaled is often very great. Certain plants are used for this reason for the drainage of wet and marshy places. The most important of these is the Eucalyptus tree.[1]
"The amount of water taken from the soil by the trees of a forest and passed into the air by transpiration is not so large as that accumulated in the soil by the diminished evaporation under the branches. Hence, there is an accumulation of water in the shade of forests which is released slowly by drainage.[1] But if the trees are so scattered as not materially to reduce evaporation from the ground, the effect of transpiration in diminishing the moisture of the soil is readily shown. It is noted, especially in case of large plants having a great extent of exhaling surface, such, for instance, as the common sunflower. Among the plants which have been successfully employed in the drainage of marshy soil by transpiration probably the species of Eucalyptus (notablyE.globulus) are most efficient."[2]
4.Assimilation.—It is not easy to find practical experiments on assimilation. Those which follow are taken from "Physiological Botany" (p. 305).
Fill a five-inch test tube, provided with a foot, with fresh drinking water. In this place a sprig of one of the following water plants,—Elodea Canadensis, Myriophyllum spicatum, M. verticillatum, or any leafyMyriophyllum(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 aparasite.[1] It has no need of leaves to carry on the process of making food. Some parasites with green leaves, like the mistletoe, take the crude sap from the host-plant and assimilate it in their own green leaves. Plants that are nourished by decaying matter in the soil are calledsaprophytes. Indian Pipe and Beech-Drops are examples of this. They need no green leaves as do plants that are obliged to support themselves.
Some plants are so made that they can use animal matter for food. This subject of insectivorous plants is always of great interest to pupils. If some Sundew (Drosera) can be obtained and kept in the schoolroom, it will supply material for many interesting experiments.[1] That plants should possess the power of catching insects by specialized movements and afterwards should digest them by means of a gastric juice like that of animals, is one of the most interesting of the discoveries that have been worked out during the last thirty years.[2]
5.Respiration.—Try the following experiment in germination.
Place some seeds on a sponge under an air-tight glass. Will they grow? What causes them to mould?
Seeds will not germinate without free access of air. They must have free oxygen to breathe, as must every living thing. We know that an animal breathes in oxygen, that the oxygen unites with particles of carbon within the body and that the resulting carbonic acid gas is exhaled.[1] The same process goes on in plants, but it was until recently entirely unknown, because it was completely masked during the daytime by the process of assimilation, which causes carbonic acid to be inhaled and decomposed, and oxygen to be exhaled.[2] In the night time the plants are not assimilating and the process of breathing is not covered up. It has, therefore, long been known that carbonic acid gas is given off at night. The amount, however, is so small that it could not injure the air of the room, as is popularly supposed. Respiration takes place principally through the stomata of the leaves.[3] We often see plants killed by the wayside dust, and we all know that on this account it is very difficult to make a hedge grow well by a dusty road. The dust chokes up the breathing pores of the leaves, interfering with the action of the plant. It is suffocated.
The oxygen absorbed decomposes starch, or some other food product of the plant, and carbonic acid gas and water are formed. It is a process of slow combustion.[4] The energy set free is expended in growth, that is, in the formation of new cells, and the increase in size of the old ones, and in the various movements of the plant.
Physiological Botany, page 356.]
[**Proofers Note: Two footnote marks [3] and [4] above in original text, but no footnote text is in the original text.]
This process of growth can take place only when livingprotoplasmis present in the cells of the plant. The substance we call protoplasm is an albuminoid, like the white of an egg, and it forms the flesh of both plants and animals. A living plant can assimilate its own protoplasm, an animal must take it ready-made from plants. But a plant can assimilate its food and grow only under the mysterious influence we call life. Life alone brings forth life, and we are as far as ever from understanding its nature. Around our little island of knowledge, built up through the centuries by the labor of countless workers, stretches the infinite ocean of the unknown.
Gray's First Lessons. Sect. VII, XVI, §2, §4, §5, §6, 476-480.
How Plants Grow. Chap. I, 119-153, Chap. III, 261-280.