Outlines of Lessons in Botany Part 11

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-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 compound.

[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 (_a.s.similation_ 4) is carried on.

"Physiologically, leaves are green expansions borne by the stern, outspread in the air and light, in which a.s.similation and the processes connected with it are carried on."[1]

[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.[1]

[Footnote 1: Reader in Botany. XI. Protection of Leaves from the Attacks of Animals.]

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.

Arrangement _Alternate_[1]

Simple or compound. _Simple_ (arr. and no. of leaflets) Venation _Netted and feather-veined_ Shape _Oval_ 1. BLADE < apex="" _acute_="" base="" _oblique_="" margin="" _slightly="" wavy_="" surface="" _smooth_="">

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 ill.u.s.trations 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 princ.i.p.ally 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 pa.s.s 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 s.p.a.ces 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]

[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 pa.s.s 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]

[Footnote 1: Concerning a few Common Plants, p. 29.]

There are many simple experiments which can be used to ill.u.s.trate the subject.

(1) Pa.s.s 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 gla.s.s 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.

[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 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]

[Footnote 1: The Essentials of Botany, by Charles E. Bessey. New York, Henry Holt & Co., 1884. Page 74. See also Physiological Botany, pp.

259-260.]

(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]

[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 gla.s.s, 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 gla.s.s, because the moisture is only given out from the green tissue, and where the ribs are pressed against the gla.s.s 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.

[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[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 pota.s.sium, iron, etc., are necessary to this a.s.similating 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.[1]

[Footnote 1: Reader in Botany. XII. Transpiration.]

"The amount of water taken from the soil by the trees of a forest and pa.s.sed into the air by transpiration is not so large as that acc.u.mulated in the soil by the diminished evaporation under the branches. Hence, there is an acc.u.mulation 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 (notably _E_. _globulus_) are most efficient."[2]

[Footnote 1: Reader in Botany. XIII. Uses of the Forests.]

[Footnote 2: Physiological Botany, page 283.]

4. _a.s.similation_.--It is not easy to find practical experiments on a.s.similation. 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 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 pa.s.s 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) Subst.i.tute 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 a.s.similative process. But,