Читать книгу An Introduction to Nature-study - E. Stenhouse - Страница 13

1. The shapes.—Make a collection of the leaves of a large number of different plants, for example, elm, beech, lime, oak, birch, ash, blackberry, pine, yew, horse chestnut, rose, holly, woodsorrel, grass. Lay each in turn flat in your notebook and trace the shape of the leaf blade by passing the point of your pencil round the edge. Measure the length and greatest width; write down these dimensions. Is the greatest width at, above, or below, the middle of the leaf blade?

Most of the leaves are flattened green plates. Those of the pine and yew are long and needle-shaped. Do you know any other leaves like these?

2. The veins.—What enables the leaf to keep stretched out? Turn it over and notice the “veins” on the lower side. Do they act like the ribs of an umbrella? Fill in the positions of the main veins in your drawings. Are the veins parallel to each other in any of your leaves? Write a list of as many leaves as you can find which have parallel veins.

3. Skeleton leaves.—Put some leaves in a saucer with a little soft water, and allow them to rot. Clean away the soft stuff from time to time by gently brushing the leaves with an old tooth-brush. Notice the “skeleton” which remains. Skeleton leaves may be made much more quickly by soaking the leaves for some time in a weak solution of bleaching powder. Wash them well before drying.

4. The colour.—Is the green deeper on the upper or the lower surface of a leaf? Which surface usually receives more light?

5. The apex.—In how many of your leaves is the apex (a) pointed, (b) blunt, (c) rounded?

6. The margin.—Examine the edge or margin of each leaf. In how many is it (a) quite plain, (b) hairy, (c) wavy, (d) saw-edged, (e) doubly saw-edged, (f) spiny? Do you find spines on holly leaves which are so high on the tree as to be out of the reach of cattle? What is the use of the spines?

7. Blackberry leaves.—Gather several leaves from a blackberry bush. Notice that in addition to the “saw cuts,” the margins of some are cut into slightly, while others are divided quite to the midrib, the leaf being thus cut into two or more leaflets. Select specimens which form a gradual series between the “simple” leaf and the “compound” leaf (consisting of three or five leaflets) and draw them.

8. The horse chestnut leaf.—Draw the compound leaf of the horse chestnut, and draw an even curved line joining the points of the leaflets. You can imagine that the compound leaf may have been formed by a leaf of this shape being cut into until it was divided into seven complete leaflets.

9. The rose leaf.—Draw an imaginary simple leaf such as may have been the original form from which the compound leaf of the rose was derived. Notice the difference in the arrangement of the main veins of the leaves of the horse chestnut and the rose. Does this account for the leaflets coming off at the sides of the midrib in the rose leaf, and springing from one point, like fingers from the palm of the hand, in the case of the horse chestnut?

10. Sycamore and ivy leaves.—If the large indentations in these leaves were continued to the midrib, would the compound leaves thus formed be of the type of the rose leaf or of that of the horse chestnut leaf?

11. The leaf-stalk.—What attaches the leaf-blade to the stem or branch of the plant? Can you see signs of the main veins joining the top of the leaf-stalk? Do you know any plant with the blades of the leaves fixed directly on the stem, i.e. without any leaf-stalks?

12. Stipules.—Examine the rose leaf again and notice the two leaf-like outgrowths at the bottom of the stalk. These are called stipules. Make a list of as many leaves as you can find which have stipules. How many leaves can you find with a sheath at the bottom of the stalk?

13. The leaf of the sweet pea.—Notice the large stipules of this compound leaf (Fig. 28). What are the tendrils? Do you think they may be mainly the larger veins of the upper leaflets? Is the leaf of the type of the rose or of the horse chestnut leaf?

14. Other compound leaves.—Compare and contrast ash, lupine, woodsorrel, strawberry, and other compound leaves with those of the rose and horse chestnut.

A leaf.—The leaves of different plants vary much in size and shape, but in general a leaf is a thin, broad, and more or less oval blade of green colour, attached by a leaf stalk to the stem or branch. In some cases, however, the leaf stalk is absent and the blade is attached directly to the stem or branch.

The veins.—The leaf is kept taut by a number of branching ribs, somewhat as the silk of an opened umbrella is stretched tightly by the ribs. The ribs or “veins” of the leaf run beneath the skin, but are generally nearer the lower surface than the upper, and are easily seen when the leaf is turned over. If a leaf is allowed to rot in a little soft water, the skin and the soft green stuff of the interior decay and leave these veins as a white “skeleton” (Fig. 23). The process may be assisted by gently brushing the leaf from time to time. A skeleton leaf may be obtained still more quickly by putting the leaf in a weak solution of bleaching powder until the skin and interior are soft enough to be brushed away. Care should be taken to use a weak solution, or the veins also will be rotted. The skeleton should be well washed in water before drying.

The arrangement of the veins in a leaf varies widely, but it falls broadly into two classes, according as the main veins run parallel or nearly parallel to each other (Fig. 24), or form a less regular network (Fig. 23). The venation of a leaf is curiously associated with the number of cotyledons possessed by the seedling; for nearly all dicotyledons (p. 23) have net-veined leaves, while the leaves of monocotyledons are almost invariably parallel-veined. Careful drawings of several typical leaves should be made, and the principal veins indicated on them.

Fig. 23.—Net-veined leaf of a dicotyledon (White Thorn). (×½)

Fig. 24.—Parallel venation of the leaf of a monocotyledon (Solomon’s Seal). (×1)

The shapes of leaves.—Although the blade of a leaf is most commonly flattened, and roughly oval in outline, there are several exceptions. The leaves of pine, spruce, larch, and yew are needle-shaped; those of grasses (Figs. 102-110) are very long in proportion to their width; while the leaves of many moorland plants are rolled up into hollow cylinders. There is some reason—could we find it—for every such variation, and the significance of some of these shapes will be referred to later (p. 47). When the dimensions of leaves are carefully measured, the proportion of the length to the width will be found to vary much in the leaves of different plants, but will be found to be pretty constant for the same sort of plant. This holds good, too, for the position of the greatest width (e.g. at, above, or below, the middle of the blade), the form of the apex of the blade (blunt, pointed, spiny, or rounded), the nature of the margin (smooth and “entire,” hairy, saw-edged, doubly saw-edged, lobed, etc.), and the extent and positions of the larger indentations. Thus, while any particular elm leaf (Fig. 124) is probably slightly different from every other elm leaf in the world, it resembles every other elm leaf more than it does any leaf from any other plant than an elm. No leaf of this shape ever grows on an oak tree or a sycamore. Thus, in spite of minor variations there is a wonderful conformity to type, and the student will find that by carefully examining the shape, venation, margin, apex, etc., of all his leaves, and above all by drawing them, he will soon be able to recognise them at sight. It is by doing this and noticing in each case the methods of folding and arrangement of young leaves in the bud that it may be possible in the future to explain some variations which are at present not understood. It has already been seen that the peculiar forms of the leaves known as cotyledons are associated to some extent with the shape and size of the seeds containing them, and with the amount, if any, of the food stored in them.

Simple and compound leaves.—Blackberry leaves (Fig. 76) well repay close examination. Some of the leaves on the bush will be found to be simple—having one blade only on the leaf stalk. Here and there, however, a leaf may be discovered which is so deeply cut into along one side, that it is almost completely divided into two leaflets; and other leaves will easily be found which consist of three or five leaflets, much resembling the leaves of the rose (Fig. 25), a near relative of the blackberry. Here, then, we have a plant which produces simple or compound leaves according to its needs. It seems as if the blackberry were still trying, as an experiment, a device which the rose tree has found so advantageous as to have adopted for good. Some other plants, the ash, for example, have compound leaves broadly similar to the rose leaf—the leaflets springing in pairs from the sides of the midrib.

Fig. 25.—Compound leaf of the Rose. L, leaflets; P, leaf-stalk; st, stipules. (×½)

Fig. 26.—Compound leaf of the Horse Chestnut. (×⅙)

The compound leaf of the horse chestnut (Fig. 26) is of a different type, for the seven leaflets all arise from one point; and the leaves of the lupine (Fig. 9) are arranged on the same plan. When the venation is examined, the reason for this becomes plain. In these cases the main veins all diverge from the top of the leaf stalk; whereas in the rose and ash the midrib gives rise to side ribs in pairs. The leaflets are naturally arranged so that one of the larger veins shall support each. The next question arising is, “What causes the differences in the methods of branching of the midrib?” At present this is a mystery. Compound leaves consisting of three leaflets are found in woodsorrel, strawberry (Fig. 50), clover, etc.

Intermediate leaves.—The ivy (Fig. 27) and sycamore (Fig. 33) have leaves which seem intermediate between truly simple and truly compound leaves. From the arrangement of the veins it is seen that they approach the horse chestnut type more than that of the rose. On the other hand, if the deep indentations of the oak leaf (Fig. 113) were carried to the midrib, the simple leaf would be divided into leaflets arranged, somewhat like those of the rose or ash leaf, along the sides of the midrib.

Fig. 27.—Ivy. (×¹⁄₁₀)

Fig. 28.—Compound leaf of Pea. Fl, flower-stalk; Sp, stipules; T, tendrils. (×½)

Stipules.—At the bases of many leaf stalks, close to the stem, are leaf-like outgrowths called stipules. They are well seen in the rose (Fig. 25) and pea (Fig. 28). Some leaves have a sheath at the bottom of the stalk, partially enclosing the stem.

Tendrils.—The pea also affords an interesting case of leaflets being modified to do special work. Here the upper leaflets seem to have remained undeveloped except for their main veins, and these have acquired a remarkable power of twining round suitable objects and so supporting the stem. Many other plants have tendrils, but these are not always modified leaflets.