Читать книгу An Introduction to Nature-study - E. Stenhouse - Страница 4

1. Preparation of the seeds.—Obtain several seeds of the broad bean, pea, mustard, yellow lupine, vegetable marrow, and sycamore; soak them in cold or slightly warm water until they are soft enough to be cut through easily with a sharp knife. The time necessary will vary with different seeds according to the size of the seeds, and with the temperature of the water. The beans should be left in the water for a few days. When the seeds are soft enough, examine one or two of each, and in the meantime put about six of each (except the mustard) in damp sawdust in a warm place. Put the mustard seeds on damp flannel in a saucer.

2. The outside of a broad bean.—Notice the flattened oval shape, with an indentation at one place (Fig. 1). What is the colour of the skin (seed-coat) of the bean seed? Is all the skin of this colour? A black scar extends along the edge from the indentation for about ¾ in. What is this scar? If beans in the pod can be obtained, see that the scar is the place of attachment of the seed stalk. Make drawings to scale, showing side and edge-views of the seed. Wipe the bean dry and then squeeze it gently. Notice that a drop of water comes out at a point at one end of the stalk scar. There is evidently a little hole here. This little hole is called the micropyle. Mark its position by a dot on the drawing.

3. The inside of a broad bean.—With a sharp knife cut the seed-coat open, beginning at the side of the seed furthest from the micropyle, and carefully remove the seed-coat. Notice that near the micropyle the seed-coat forms a funnel-shaped depression, and that the point of the funnel is at the micropyle. Does anything fit into the funnel? A little cone may be seen to fill the funnel; this conical body is called the radicle. Make a drawing of the seed after the removal of the seed-coat. Look at the edge opposite the radicle and notice that a crack divides the body of the seed into halves. Put the point of your knife blade into the crack, and gently force the halves apart. They come apart without tearing, showing that they are naturally separate, although they fit so closely together.

These two swollen bodies are called the cotyledons. Separate them and see, at the point where they join the radicle, a little curved rod, evidently a continuation of the radicle, lying between them. This rod is the plumule. Take off one cotyledon, and make a drawing of the inner face of the other cotyledon, with the adhering plumule and radicle (Fig. 2).

4. Starch present in the cotyledons of the bean.—Scrape the inner surface of a cotyledon and then pour on it a drop of iodine solution.[2] Is there any change? Pour also a drop of iodine solution on a piece of laundry-starch. Is a similar blue colour formed? What substance is probably present in the cotyledons of the bean?

5. The pea.—Examine a pea in a similar manner. Make drawings showing the stalk-scar, the micropyle, and the plumule and radicle with their manner of connection with the cotyledons. Does the end of the radicle point towards the micropyle? How many cotyledons has this seed? What shape and colour are they? Do they contain starch?

6. The seed of the yellow lupine.—Compare this with the bean and the pea, and find out how many cotyledons it has, and whether they contain starch. Can you find the plumule? It is very small, but occupies a position similar to that of the plumule of the bean. Does the end of the radicle point to the micropyle?

7. The vegetable marrow seed.—Notice the peculiar shape (somewhat like a pocket-flask) of the seed, and the thickened margin which runs round it. Carefully cut the seed-coat away so as not to injure the part inside. How many cotyledons are present? What is their colour? Do they contain starch? Can you see the plumule and radicle clearly? If not, do not decide that they are absent, but leave the question to be settled later, when you watch a vegetable marrow seed “come up.”

8. The mustard seed.—Notice how much smaller this seed is than the others. With a balance, find how many mustard seeds are equal in weight to one bean seed. Observe the stickiness of the seed-coat of the soaked seed, and then remove it carefully with needles, exposing two thin plates, each one folded on itself, and one tucked inside the other, like two sheets of note-paper. These are the cotyledons; it seems that the smallness of the seed may be mainly due to the small size of the cotyledons. What is their colour? Remember these characters and try, when you watch the young plants come up later, to find an explanation of them.

9. The sycamore fruit.—The seed of the sycamore is enclosed in a case which has a wing attached to it. The wing, the case, and the enclosed seed together constitute the fruit of the sycamore. The fruits occur in pairs (Fig. 137). Notice that a cord runs out to each fruit from the stalk on which the pair of fruits is borne. Make a drawing of a pair of fruits, then separate the fruits.

10. The sycamore seed.—Cut open a fruit. Can you see anything between the seed and the fruit-case? Would the hairy covering of the seed tend to keep it warm during the winter? Why? Why do you prefer to wear flannel in winter and linen in summer? Flannel is more fluffy than linen.

Remove the seed-coat carefully. Running down one side you will see a little curved rod. This is the radicle. Gently raise it with the point of your knife. Notice that the rest of the seed seems to consist of a green part, which is curled up. Uncoil the curls carefully. You find that they are two green leaves, fixed at the top of the radicle. These are the cotyledons. In the seed each cotyledon is first folded in two across the middle and then coiled up. Make a sketch showing the coils (Fig. 4). Can you see the plumule? It is just at the top of the radicle, where the cotyledons are fixed on.

Plants are living things.—One of our foremost naturalists[3] tells us that when he goes out into the woods, or into one of those fairy forests which we call fields, he finds himself welcomed by a glad company of friends, everyone with something interesting to tell. Such a feeling would be quite impossible to one who did not vividly recognise the fact that plants are alive; for it is precisely this recognition or its absence which makes the observation of the forms and habits of plants fascinating or the reverse. Let the Nature-Student, then, at the outset of his work, keep the idea of life inseparably bound up with his every thought about plants. It may at first require a little effort, but before long it will enable him to understand how the friendship of the more silent half of animate nature may form one of the great pleasures of life.

The study of seeds.—The manifestation of life is so striking, and the changes in form and size take place so rapidly, in the germination of seeds, that the study of plants cannot better be commenced than with this stage of their growth. The method has also the logical virtue of beginning at the beginning, or nearly so.

These early changes can be well observed by taking various common seeds, soaking them in water until they are soft, and then allowing them to germinate in damp sawdust, taking a few out at intervals and noting their progress. The growth of the seeds takes place more rapidly if they are kept in a warm room, but in any case some days will probably elapse before much change is noticeable in them.

During the interval of waiting, some of the seeds themselves should be carefully examined, and drawings of all the parts should be made. The drawing ought on no account to be omitted. It compels the student’s attention to details which would otherwise pass unnoticed; and a careful sketch is a much better record of an observation than any amount of description alone could be. The drawing need not be elaborate; an outline pencil-sketch to scale will usually be sufficient.

Fig. 1.—A Broad-Bean seed. A, side view; B, edge view: st. sc., stalk-scar; m, micropyle. (×⅔.)

The seed of the broad bean.—The seed of the broad bean (Fig. 1) is large, having a diameter of perhaps an inch and a half, and a thickness of half an inch. In shape it is oval, but at one region the edge is indented, and a black scar (st. sc.) runs from the indentation along the edge for a distance of about three-quarters of an inch. This scar is the place of attachment of the stalk which formerly carried the seed in the bean-fruit (pod). It may be called the stalk-scar. If a soaked bean is wiped dry and then gently squeezed, a small drop of water escapes from the end of the stalk-scar nearest the indentation. The hole out of which the water comes is very small and difficult to see, but its position is thus made clear. This hole (m) is called the micropyle,—a word meaning the “little gate.”

The bean seed is covered by a tough brown skin, the seed-coat (Fig. 2, s.c.), a funnel-shaped depression in which leads to the micropyle (m). The depression is occupied by a part of the seed which is shaped like a conical peg and called the radicle (R); the point of the radicle is directed toward the micropyle. The great body of the seed is composed of two fleshy, cream-coloured lobes, easily wedged apart by inserting a knife-blade between them; these fleshy lobes are the cotyledons (Cot.). Between them, and continuous with the radicle, is a small yellow body, the plumule (pl.). The relations of the radicle, plumule and cotyledons are best seen by removing one cotyledon (Fig. 2).

Fig. 2.—Broad-Bean seed, seen from the inside, after the removal of half the seed-coat and one cotyledon. Cot., the inner face of remaining cotyledon; C′, area of attachment of other cotyledon; m, micropyle; pl, plumule; R, radicle; S.c., seed-coat; st. sc., stalk-scar. (×1.)

A scraped cotyledon at once turns blue when a drop of dilute iodine solution is poured on it, thus showing the presence of starch. We shall see in Chapter II. what use the growing seedling makes of the starchy food which is stored in its cotyledons.

The seed of the pea.—Except in size and shape the seed of the pea is very similar to the bean seed. Its form is spherical, and the scar left by the stalk which formerly attached it to the wall of the pea-pod (Fig. 3) is plainly to be seen. Pointing towards the micropyle is the peg-like radicle; the plumule lies between the hemispherical cotyledons. As before, the cotyledons can be proved to contain starch, by the blue colour which is formed when a drop of iodine solution is poured on the scraped surface.

Fig. 3.—Pods and Seeds of Pea. (× ½.)

The seed of the yellow lupine.—The seed of the yellow lupine is about as large as a pea, but it is slightly flattened in shape. The seed-coat is prettily mottled; when it is removed, the greater part of the seed is found to consist of two cotyledons. They are somewhat swollen, but the stored food is not starch. The plumule and radicle occupy positions similar to those of the bean and pea.

The vegetable marrow seed.—This seed has a rather curious shape, and somewhat resembles a pocket-flask. It is flattened, and the border of the seed-coat is thickened and of silky appearance, the rest of the “skin” having some resemblance to kid. The two cotyledons, which compose the greater part of the seed, are white and only slightly fleshy. The plumule and radicle are at the pointed end of the seed, and are difficult to see.

The mustard seed.—In comparing the mustard seed with those already described, one is struck with the great difference in size. An average broad-bean seed weighs about 600 times as much as the mustard seed. While the two fleshy cotyledons make up the bulk of the seed of the bean, pea, lupine and vegetable marrow, the cotyledons of the mustard seed are thin and leaf-like. They are folded on themselves, one inside the other (as at g, Fig. 61), and enclose the radicle. The characters of the cotyledons account very largely for the small size of the mustard seed. It will be seen, when the growth of the young plants is watched, that the difference is associated with the special duties which the cotyledons perform in the various cases.

Fig. 4.—Sycamore Fruit, cut through in the plane of the wing. s.c., seed-coat (indicated by a thick, broken line); f.w., fruit-wall; h, layer of fine hairs; R, radicle; pl., plumule; cot. 1, cot. 2, cotyledons (diagrammatically shaded). (×2.)

The sycamore seed.—What is generally called the seed of the sycamore is really a fruit. The fruits are in pairs (Figs. 33 and 137), and each half consists of a flat wing and a rounded case in which the seed itself is enclosed. The round seed-cases of the two fruits are connected together. When they come apart, a scar marks the place where they were formerly in contact, and a little cord runs out to each fruit from the stalk on which the pair of fruits is borne.

Between the sycamore seed and the wall of its case is a layer of fine hair (h, Fig. 4), which forms a warm nest for the seed in winter. The seed is surrounded by a thin, brown seed-coat, and consists mainly of two cotyledons, but these are very different from any yet described. Each is a green leaf, measuring, when unfolded, about an inch in length. It is first folded across the middle of its length, and then rolled up into a close coil with its fellow. The coils are very plainly to be seen when the seed coat is removed, or when the whole seed is cut through, by a sharp knife, in the plane of the wing. Running down one side of the seed is a green rod, the radicle (R, Fig. 4). The two cotyledons (cot. 1 and cot. 2) spring from its upper end, and between them is the tiny plumule (pl.)

The sycamore seed bears more resemblance to the mustard seed than to the others, but it is on a much larger scale. In each of these two seeds the cotyledons are plainly leaves, while in the others their nature is disguised by the great accumulation of stored food in them.