Читать книгу An Introduction to Nature-study - E. Stenhouse - Страница 17

1. A typical bud.—Split a cabbage, or a lettuce “heart,” down the middle, and observe how the leaves are arranged round the conical end of the stalk. The leaves which are fixed lowest on the stalk are the largest, and they cover the outside of the bud. The leaves are smaller and smaller as they are fixed nearer and nearer the end of the stalk, until round the tip they are almost too small to be recognised as leaves. A bud is the tip or “growing point” of a stem or branch, together with the young leaves which crowd round it. Draw the section.

2. A sycamore twig.—Look in the axils (p. 47) of the leaves of a sycamore twig in summer or autumn, and notice the buds. Can you see any buds on the lower parts of the twig—which bore leaves twelve months or longer ago? Did these also arise in the axils of leaves? How can you be sure they did? Tie a piece of string, or tape, round this twig and look at it often, making a note of every change which you see in it.

3. The fall of the leaf.—On what date did you first notice that the leaves of the sycamore were falling? Did the first leaves fall from the twigs at, or near, the ends of the branches, or lower down near the trunk? Examine the scar left by the first leaf which falls from your twig. What is its shape? What is the meaning of the dark dots on the scar? Gently take off the leaf nearest to the scar. Does it come off easily? Are there any dark dots on this scar? Can you see anything on the end of the leaf stalk corresponding to the dots? Do you think the dots are the ends of the food pipes referred to on p. 53?

Make a drawing of a fallen leaf and write down on the drawing at the proper place, the colour of that part of the leaf, or better still, colour the drawing with water-colour paints. Collect good specimens of the autumn leaves of as many other trees as possible, and make coloured drawings of them.

On what date did you first notice that all the leaves were shed? Make similar notes for other trees.

4. The structure of a sycamore bud.—In winter take off a sycamore twig which has a big bud at the end, and examine it. The bud is clothed with overlapping scales of a pale green colour. Make a drawing of the bud, double the size, and then take off the scales one by one and count them. Look with a lens at the upper end of one of the largest scales. Can you see any trace of a rudimentary leaf blade? When the last scale is removed there remains on the end of the twig a tiny tuft of delicate green leaves surrounded by a little down. Count them, and notice how each is folded up fanwise. In some buds you may find a bunch of green flowers in the middle. These buds are larger than those containing leaves only.

Cut a bud across with a sharp knife, and examine the cut surface with a lens to see more clearly how the scales and young foliage leaves are arranged. Draw what you see.

5. How the bud bursts.—In spring watch your twig very carefully to be sure you do not miss any stage of the opening of the buds. Make drawings every day or two of the end bud, when it has begun to unfold. Notice how the scales fall off—those on the outside first—and how the foliage leaves push open the upper scales and come out. Watch the way in which each leaf opens its folds and spreads itself out flat.

The buds may be made to open earlier by cutting off a twig about two or three feet long and keeping it in water in a warm room.

6. Starch is stored up in the twigs.—Cut off a twig and pour a drop of iodine solution on the cut end. What does the blue colour indicate?

7. The growth of the bud.—Watch your bud growing, and notice that the tip of the twig—which was surrounded by the young leaves—elongates so that each pair of leaves is soon separated from the next pair. Notice the rings of scars which the scales left when they fell off.

8. A year’s growth.—Look down the twig until you find the rings or scars which were left last spring, when the scales fell off the expanding terminal bud. A year ago the end of the twig was at this point, so that the length between one such set of rings and the next marks a year’s growth.

9. Side branches.—The buds on the length formed last summer but one may have grown out into side twigs. Try to find the leaf scars below each of these side twigs.

10. A horse chestnut twig and its buds.—Examine in the same way a horse chestnut twig and trace its history. The buds are larger than those of the sycamore, and each is covered with a shining, sticky layer of resin. What do you think is the use of the resin? Put a bud in water, and when you take it out notice how the water runs off and leaves the bud dry.

Pull one of the terminal buds to pieces. The scales will come apart more easily if the bud is soaked for some time in methylated spirit, to dissolve out the resin. Notice the thick layer of down inside the scales. What do you think is its use? Scrape the down gently, and carefully clean it away from the little foliage leaves in the middle. See how the leaflets of each leaf are folded. In some terminal buds you may also find a little pink flower-spray.

Tie a piece of string round a growing twig, so that you can recognise it, and watch all the stages of the expansion of the terminal bud, the unfolding of the leaves, and the elongation of the tip (Figs. 37, 39 and 40). Cut off other twigs two or three feet long, in February or early March, and keep them in water in a warm room.

11. Other buds.—Examine the buds of the beech, lilac, violet, dock, fern, etc., and make drawings showing (1) how the leaves are arranged with regard to each other, and (2) how each leaf is folded or rolled. As a rule these points can be easily made out by examining with a lens the cut surface of a bud which has been cut across with a sharp knife; but the buds should also be examined at frequent intervals when they are unfolding.

A typical bud.—An excellent idea of the structure of a typical bud can be obtained by splitting down the middle an ordinary cabbage, or a lettuce “heart,” and examining the manner in which the leaves crowd round and cover the conical end of the stalk. Around the tip or “growing point” the leaves are extremely small and tender. They are overlapped by slightly larger leaves, which spring from the stalk a little lower down. These in their turn are covered by still larger leaves, inserted at a yet lower level, and so on—the largest and oldest leaves folding over the smaller and more recently formed. The growing point of a stem, or of a branch of a stem, surrounded by a cluster of immature leaves, is called a bud.

The history of a sycamore twig.—The student who would know the meaning of the various marks and scars on the surface of a twig, should select one and follow carefully for a year the fate of the buds which it bears. It is convenient to begin by studying a twig on a sycamore tree. It may be marked for ease of recognition by tying a piece of tape on it. If several students are working, each should write his name or number on a luggage-label and fix this to his twig.

Fig. 33.—Sycamore leaves and fruits.

(From a photograph by Mr. A. Flatters.) (×¹⁄₁₂.)

The positions of the buds.—In summer the younger twigs of the sycamore tree are covered with large five-pointed leaves (Fig. 33). The leaves come off in pairs, each pair being at right angles to the pair next above or below. Every leaf is engaged throughout the day in building up—by means of the green stuff in its interior—starch and other foods (p. 50), and in giving off excess water, in the form of invisible vapour, through its stomata (p. 53). In the axil (p. 47) of each leaf is a little bud, called from its position an axillary bud (Fig. 41, B), and at the very tip of the twig is a larger terminal bud.

Autumn colours and the fall of the leaf.—As the summer wanes, the soil becomes colder, and the chilled roots lose much of their power of absorbing moisture. It is plain that if the leaves continued giving off water when no fresh supplies were forthcoming the tree would suffer. How is the danger to be met? Starch and other foods have already been stored up in quantity sufficient to supply the needs of the winter and the early spring. The leaves have finished their work, and one by one they fall off. But this does not take place until careful preparation has been made. Their green colouring matter breaks up; the part which may still be useful to the plant drains into the stem, leaving little heaps of yellow grains in the leaves. Or a special colouring matter may be formed, which, united in various ways with the materials of the dying leaf, gives the warm shades of red, orange, and purple which make the woods so beautiful in autumn. When all is ready, a layer of cork (Fig. 41, C) forms at the junction of the leaf stalk and the twig so that no raw wound may be left; the leaf-base splits across, just above the cork layer, and the leaf flutters to the ground, there to rot and make rich leaf mould.

The leaf scars.—The former position of each leaf is now marked by a curved scar (l.s. Fig. 34), and a row of brown dots (v.b.) in the scar still shows where the food-pipes bent outwards from the twig into the leaf. A line (a) stretches across and joins the two scars of each pair.

The buds.—Just above every scar is the bud which arose in the axil of the fallen leaf. It is covered with overlapping scales of light green colour. At the end of the twig is a single terminal bud, similar in appearance to the axillary buds, but of larger size. It is instructive to take the terminal bud of a twig to pieces. The outer scales are tough and green, while the inner ones are thinner and have a beautiful silvery appearance. Usually there are fourteen scales. Each is long and narrow and bears at its upper end the rudiment of a leaf blade, which cannot usually be seen well without a lens. The scale is really a leaf which has been arrested in its development.

Fig. 34.—Sycamore twig in winter. l.s., leaf-scars; v.b., ends of food-pipes; R, rings of scars left by scales of last winter’s terminal bud. (Slightly reduced.)

Fig. 35.—Cross section

of a Sycamore bud. (×7.)

When all the scales have been removed, there remains a tiny tuft of delicate green foliage leaves surrounded by a little down. Each leaf is folded fanwise, both for convenience of packing, and to protect its tender tissues from the cold and the damp when the bud is expanding. The complicated folding of these leaves is well shown in Fig. 35, which is a magnified sketch of a cross section through a sycamore bud. The large sheaths surrounding the young leaves are four of the overlapping scales. In a large terminal bud a bunch of green flowers may often be found.

The bursting of the buds.—About the middle of April the tree wakens to new life. The stored food makes its way to the terminal buds; invigorated by the rich sugary sap the young leaves swell and push forward, burst apart the scales, and open out their folds to the light and air, as if eager to get to work at the earliest possible moment. The scales fall off to the ground, leaving close-set rings of scars; the growing point elongates, and new leaves—which were indistinguishable in the bud—grow out and expand. During the summer, food is plentiful, and a little bud appears in the axil of every leaf. Only with the autumn does the activity of the tree slow down. Except for some two pairs at the tip, the newest leaves now remain stunted. They form scales and close round the tender growing point protectingly, in readiness for the winter.

Fig. 36.—Sycamore twig in winter. (×⅛.)

If necessary, the axillary buds could have behaved as the terminal bud did, in which case they would have grown out into side twigs. They usually remain small, however, until next year (Fig. 36), for the leaves are so busy making food, and the terminal bud is so busy growing in length, that no energy can be spared for their further development. If any accident had befallen the terminal growing point, one or more of the axillary buds would have grown out into side twigs. Gardeners take advantage of such reserve buds when they clip off the ends of twigs to make a plant grow “bushy.”

Fig. 37.—Horse Chestnut twig in winter. (×¹⁄₁₁.)

Fig. 38.—The terminal part of a Horse Chestnut twig in winter. l.s., leaf-scars; v.b., ends of food-pipes. (×⅔.)

A year’s growth.—When the bud scales drop off they leave, as we have seen, a series of closely-set rings of scars. The distance between one set of rings and the next (as at R., Fig. 34), therefore represents a year’s growth. The student should get a twig two or three feet long and find out for himself, by examining the marks on it, what the twig did last year, two years ago, and three years ago. With care he will be able to say definitely in which year any fairly recent side-twig began to grow out from its bud.

A horse chestnut twig.—A winter twig of horse chestnut (Fig. 37) is very similar in its general features to what we have seen in the sycamore. The buds are in pairs at right angles to each other, and below each bud is a large corky leaf scar (l.s. Fig. 38), with the positions of the former food-pipes marked by black dots (v.b.). These buds, however, are larger than those of the sycamore, and each is covered with a shining layer of resin, to keep out insects and the rain.

We may slip the end of a penknife under the bud scales and remove them one by one. The first few are thin and papery, and soaked in resin. Those next inside are woody and much thicker. Next comes a layer of papery scales, inside that a coat of cottony down, then another soft papery layer, and lastly a thick pad of down. When we carefully scrape away this down, we find—warm and cosy in its midst—a tuft of little objects with a most quaint resemblance to hands clad in woollen gloves. We remove one of these, and on scraping it gently with a knife we see that the “hand” has seven fingers; and it presently becomes clear that each finger is a tiny green leaflet folded on itself, and that the hand is a young leaf. If we take off these gloved leaves in turn one by one, we find as we proceed that they become smaller, until they are almost too small to be distinguished in their fluffy nest. And when all the down and the baby leaves are scraped away, the tender growing point of the twig is left alone at the summit of a little cone, with steps showing where the leaves were.

Had the twig been left undisturbed on the tree, the bud would have awakened in spring and begun to grow (Fig. 39).[7] The scales and the down would have been shed, leaving only the rings of scars as a memento of the winter sleep; the growing point would have pushed on and on, lengthening perhaps a foot or more in three weeks; the leaves would have opened their bright green fingers to the spring air, and begun their work (Fig. 40), only to cease when in the autumn they too dropped off, and the new buds tucked themselves up in their beds to go to sleep. The leaves of the horse chestnut fall off, as do those of the sycamore, owing to a “separation-layer” arising at the base of the leaf stalk (Fig. 41), and in this case each leaflet also becomes separately detached in the same way.

Fig. 39.—Later stage of the Horse Chestnut twig of Fig. 37. (×¹⁄₉.)

Fig. 40.—The later development of the terminal bud of the twig of Figs. 37 and 39. (×⅕.)

Other forms of buds.—Surrounding the young silky-fringed leaves of the beech bud are several crimson membranous scales which are really the stipules (p. 43) of undeveloped leaves. The thin, soft parts of the leaf blade are sharply pleated (Fig. 42) between the side veins which spring from the midrib.

The way in which the branching of a tree depends on the buds is well shown in a lilac. The terminal bud does not usually develop, so that each of the two lateral buds just below grows out into a branch, producing the characteristic “forking.” In the lilac bud every gradation between scales and ordinary foliage leaves may be seen.

Fig. 41.—Longitudinal section of twig of Sycamore. A, base of leaf; B, axillary bud; C, cork layer. (×2.) (From a photograph by Mr. A. Flatters.)

Fig. 42.—Cross section through leaf-bud of Beech. (×6.)

In the violet bud the two margins of the leaf are rolled inwards towards the midrib; while in the dock they are rolled backwards.

Young fern leaves (Fig. 146) are not folded from side to side like the examples referred to above, but are rolled into a tight coil from apex to base. It is the upper surface of the leaf (frond) which is to the inside. As the leaf grows, the coil straightens out.

EXERCISES ON CHAPTER IV.

1. Where and when are the buds of common English trees formed? (1901)

2. Show, by describing and drawing one example, that the branch of a tree may preserve a record of past seasons in the bark. (1901)

3. Draw an unopened bud of sycamore. Of what parts is it composed, and how are the parts arranged? (1901)

4. What can be seen inside a large bud of horse chestnut? On what part of the branch are the largest buds to be found? (1898)

5. Upon what does the method of branching of a tree depend? Give examples.

6. Examine pollarded trees; what is the cause of their peculiarities?

7. Examine buds of various trees in spring, and try to discover in which cases the bud scales are (a) modifications of entire leaves, (b) modified leaf stalks, and (c) modified stipules.

8. Examine and describe the method of protection, during the summer, of the axillary buds of the plane.

9. Take a poplar shoot during late summer, and examine the corky separation-layer forming at the base of each leaf. Write an account of its use, and make drawings of its appearance.

10. The “heart” of a cabbage or lettuce is of lighter colour, sweeter taste, and more tender texture than the external leaves. How do you explain these differences? (King’s Schol., 1902)

11. Describe and explain the effect of clipping a privet hedge.

12. Give an account of the changes in appearance in any common leaf during the whole period of its growth. Explain briefly what part the leaf plays in the life of the plant. (Certificate, 1904)

13. Explain precisely how you would decide whether a given specimen consisted of (a) one compound leaf, or (b) a twig bearing several simple leaves.