Читать книгу An Introduction to Nature-study - E. Stenhouse - Страница 22

1. A creeping stem.—Examine a plant of the ground ivy. Is the stem strong enough to stand upright? How does it spread out its leaves to the light and air? The stem grows along the ground, and at intervals it gives off a pair of leaves which grow upwards, and a tuft of roots which grow down to the ground.

2. A runner.—Is the “runner” of the strawberry of the same nature, i.e. is it a continuous stem like that of the ground ivy? When the plant is carefully examined, the creeping “stem” is seen to be a branch arising in the axil of a leaf. The branch runs along the ground for a little distance, and then roots itself and gives off a number of leaves. In the axil of one of these another branch arises and runs on in the same direction. The same branch does not run on and on.

3. A stolon.—Follow carefully the underground part of the couch grass and make out its connection with the main shoot. It is a branch like the runner of the strawberry, which arises in the axil of a leaf, and extends only to the next shoot.

Compare the stolons of the cinquefoil.

4. A potato tuber.—Examine a potato tuber (the part which is eaten). Notice the “eyes.” These are buds, with scale leaves. Leaves never occur on roots, so that the potato must be an underground stem. Put a pin into every eye, and wind a thread round the tuber along the bases of the pins. It forms a spiral. Cut the potato into halves, and pour a drop of iodine solution on the cut surface. What is the meaning of the blue dots which at once make their appearance? Plant a potato in warm, moist earth, and when it has sprouted notice that each bud (eye) has given rise to a branch.

5. Bulbs.—Cut an onion or snowdrop bulb down the middle, and draw what you see, marking on your drawing the outer scale leaves, the swollen bases of last year’s leaves, the young leaves in the middle, the short, thickened stem, and the roots. Also cut other bulbs across and again draw. What is the similarity and what is the difference between these bulbs and such a bud as a cabbage?

Also examine hyacinth, tulip and daffodil bulbs. Put them in glasses with water touching their bases, and watch them grow. What do they live upon?

6. A crocus corm.—Obtain a few crocus “bulbs” in the early winter. Observe the tough outer tunic springing round the edge of a circular scar on the base. If there are any roots they come off from the scar. Take off the tunics from one “bulb” and observe the bud or buds at the top of the white mass inside. Other tunics cover the buds. Cut lengthwise through the mass of the “bulb” so as to bisect the largest bud. Separate the parts of the bud with a needle and notice (a) the thin outer leaves, (b) the young foliage leaves, (c) the flower-sheath and flower. Pour a drop of iodine solution on the cut white mass (the stem) below the bud. It turns blue. Why?

What is the principal difference between a crocus “bulb” and the bulb of an onion, hyacinth, or tulip? A true bulb is mainly composed of swollen leaves or leaf bases; in the crocus the thick, rounded stem makes up most of the bulk. It is better, therefore, to speak of a crocus corm, to indicate the difference.

Plant the remaining corms and examine them at intervals for a year. Notice the formation of the roots, the lengthening of the buds, the formation of the flowers, the activity of the foliage leaves after flowering (why?), the withering of the roots and leaves in summer, and the growth of the enlarged base of the branch into next year’s corm.

Creeping stems.—Instead of climbing, many stems find that the best method of spreading out their leaves is to creep along or under the ground, and give off leaves and roots at intervals. Not only does this device prevent the leaves of one node from interfering with the light and air supply of those of the next, but the plant is continually coming in contact with a fresh lot of soil. The ground ivy is an instructive example of this method of growth. The stem creeps along the ground, and at every node it gives off a pair of leaves which grow upwards, and a tuft of roots which grow down into the ground.

Fig. 50.—Runner of Strawberry. (×⅓.)

The runner of the strawberry (Fig. 50) appears at the first glance to grow in a similar manner. As a matter of fact, however, the apparent stem is a branch arising in the axil of one of the leaves of the last node. The branch runs along the ground and gives rise to a new shoot, and from this another branch, springing from the axil of a leaf, forms another runner. The same branch does not run on from shoot to shoot.

The stolon of the couch grass (Fig. 51) is somewhat similar. The erect stem of the plant is divided, as usual, into nodes or knots (from which the narrow, sheathing leaves arise) and internodes. Branches (stolons) spring in the axils of the lower leaves, turn downwards, and run on underneath the soil, taking root again at some distance from the parent plant.

Fig. 51.—Stolon of Couch Grass. (×⅙.)

Fig. 52.—Creeping underground stem of Solomon’s Seal. a, bud of next year’s aërial growth; b, scar of this year’s growth; c, d, e, scars of aërial growth of previous years; w, roots. (×¾.)

Underground stems.—Although the stem is usually that part of the axis of a plant which is above ground, there are many exceptions. The bracken fern, daisy, coltsfoot, Solomon’s seal (Fig. 52), and many other plants have stems which are ordinarily buried in the ground, giving off leaves above and roots below. In some cases such underground stems become much swollen with stored food-material—manufactured by the leaves in excess of immediate requirements. In the potato, for example, certain underground branches of the stem store up starch to such an extent that their ends become fleshy, ovoid masses some inches in thickness (Fig. 53). The true nature of these tubers is revealed by the buds or “eyes” which spring upon them. The buds are arranged spirally—as may be seen by sticking a pin into each and joining up the pins with thread—and when the tubers are kept in a warm, moist place, each bud grows out into a new leafy shoot.

Fig. 53.—Part of a Potato plant, showing the

old tuber (dark) and several new ones. (×⅙.)

The structure of a bulb is easily made out in the onion, tulip (Fig. 54), hyacinth, or daffodil. When such a bulb is cut down the middle, it is seen to be mainly composed of leaves or leaf-bases, swollen with stored food. Inside these are the young leaves and the flower bud, which would have expanded next season; and on the outside are a few thin scale leaves. All these leaves spring from a fleshy button at the base, which gives off roots below. The button is the flattened stem. When a plant produces a bulb it will generally be found that it flowers either very early or very late in the season; that is, at a period which would not be very favourable for the work of the leaves. The flower (Fig. 55) is produced at the expense of the stored food in the bulb—made in excess of the requirements of the previous season. After the plant has flowered, the new leaves work until they have made enough food for next season’s flower, and then they also die.

Fig. 54.—Longitudinal section of Tulip bulb. Fig. 55.—Daffodil. (×¹⁄₉.)

zk, modified stem; zs, scale leaves; v, terminal bud; k, young bud; w, roots. (×1.)

The so-called bulb of the crocus is technically known as a corm (Fig. 56). It differs from a true bulb in consisting mainly of a fleshy, rounded stem in which the surplus food made by last year’s leaves is stored up. The plant is thus able to flower early, without waiting for the new leaves to supply food. The swollen stem bears one or more buds, and the whole is surrounded by tough tunics of scales. When the corm begins to grow, roots are put out from the base, and the flowers and—later—the leaves of the buds expand. The foliage leaves continue their work after flowering, and the food which they make accumulates in the base of the former bud, which becomes swollen to form the new corm for next year’s flower. The leaves then die down, their bases becoming the tunics of the new corm.

Fig. 56.—Crocus corm, seen from the side, from below, and in longitudinal section. c′, base of bud, which will grow into next year’s corm; fd.ch., food channel; flr, flower bud; l, young leaves; rts, roots; t₁, t₂, t₃, t₄, tunics. (×½.)

EXERCISES ON CHAPTER V.

1. Mention experiments, which prove that organic substances are formed in the leaves, and distributed to other parts of a green flowering plant. By what channels are they distributed? (1895)

2. Draw a cross section through the stem of a flowering plant selected by yourself. Explain the uses of the chief things seen in the section. (1897)

3. Mention experiments or observations which show by what tissues water ascends to the leaves, and nutritive substance descends from the leaves. (1897)

4. By what tissues does water pass along the stem of a tree to the leaves? Give proofs of your statements. (1898)

5. Describe the effect of a tight ligature upon a growing hazel stem. (1898)

6. What proofs can be given that the stem of a tree draws nourishment from the leaves? (1898)

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

8. Mention an experiment which shows that organic substance formed in the leaves travels down the stem outside the cambium. (1901)

9. Obtain thin sections (e.g. plane-shavings) of as many different kinds of wood as possible, and gum them into a book, writing under each section the name of the wood and the direction (transverse, radial-longitudinal, or tangential-longitudinal) of the section.

10. Make yourself familiar with the appearance and characters of the different kinds of timber used in carpentry and joinery.

11. Notice the light-brown spots on the bark of twigs of apple and horse chestnut. These are called lenticels; they are breathing-pores. In how many other trees can you find lenticels?

12. Observe whether an ivy stem puts out roots only where they can become fixed to a support, or indiscriminately.

13. Make a list of plants which you have observed to climb by twining stems, and note whether they are clockwise or counter-clockwise climbers.

14. Make careful drawings of all the tendrils you can find, and try to discover which part of the plant has been modified to form the tendril.

15. Gently stroke a tendril of the passion flower and write an account of any resulting movement.

16. Mention any three climbing plants which grow wild in this country, and explain in each case how the plant climbs. (1895)

17. What is the difference between (a) a thorn, (b) a leaf-spine, and (c) a prickle? Give examples. (1896)

18. Enumerate and briefly describe the principal varieties of tendrils, and explain how they act. (1897)

19. If a wire is fastened tightly about a growing branch of a common tree, and left for two or three years, what effect will be produced, and how can the effect be explained? (1904)

20. What processes of vegetable growth are accompanied by the presence of sugar? Give examples from plants within your own experience. (King’s Scholarship, 1904)

21. What are the chief uses of the vessels of a herbaceous stem? Mention observations and experiments in support of your statements. (1905)

22. How can you demonstrate experimentally that food substances, formed in the leaves of a tree, descend to the branches below? (1905)