Читать книгу An Introduction to Nature-study - E. Stenhouse - Страница 20

1. The formation of wood.—(i.) In summer take a horse chestnut twig of three or four years’ growth. Cut through it with a sharp knife at the following places, and trim the cut ends flat:

 (a) Near the apex;

 (b) at the middle of the current year’s growth;

 (c) near the bottom of the current year’s growth;

 (d) about the middle of last year’s growth;

 (e)”” the previous year’s growth.

Make a drawing of what you see in each case:—In (a) the twig is covered on the outside by a green skin. In the middle is the soft pith. Between the two is a ring of separate strands. In (b) and (c) the strands have joined up, and a distinct, though thin, layer of wood surrounds the pith. Bast and other soft tissues lie between the wood and the bark. (d) has two layers of wood. (e) has three layers of wood.

(ii.) Split each length longitudinally. Why is it easier to do this than to cut the twig across? In which direction does the grain of the wood run? Make out in each piece the pith, strands, or layers of wood, bast, etc., and skin or bark. You can tear off the bast in ribbon-like shreds. See how the strands run out into the young leaves. Cut lengthwise through the junction between the main twig and any side twigs, and notice that corresponding parts are continuous.

2. The strength of a grass stem.—Notice the relatively enormous strength of a straw and other grass stems.

Burn a straw and observe the tube of mineral matter which is left behind. Examine a piece of bamboo; is it hollow or solid?

Woody stems.—To enable them to bear the weight of the leaves and branches, and to withstand the force of the wind, the stems of plants are strengthened in various ways. Most commonly this is effected by the formation of wood in the walls of the water-vessels.

Even in succulent stems, such as that of the sunflower, the strands of vessels are stiffened by the long and narrow wood pipes which run along them; and when the strands join up to form a complete cylinder a very strong column is the result. Engineers make use of the same device, knowing that the same amount of material will bear a far greater stress when made into a hollow cylinder than it will in any other form.

The thickening of woody stems.—In dicotyledons (p. 23) and gymnosperms (p. 163) the cylinder, which is formed by the joining-up of the conducting strands, consists of three layers. The innermost of these is the wood, and the outermost is bast. Between them is a very delicate layer called cambium, which is continually dividing and forming more wood on its inner side and more bast on its outer side. The wood is hard and resists pressure, while the bast is soft and is squeezed against the inside of the bark by the expanding wood (Fig. 43).

The formation of new wood and bast takes place vigorously during the summer, at the expense of the food which is manufactured by the leaves and travels along the bast to the active cambium. As autumn comes, the activity of the tree slows down, and the new wood is formed of closer texture. In winter the process stops altogether; but with the warmth and the plentiful food-supply of spring the formation of new, open-textured wood is resumed.

Fig. 43.—Portion of a four-year-old stem of the Pine, cut in winter. 1, 2, 3, 4, the four successive annual rings of the wood; b, bast; br, bark; c, cambium; f, spring wood; i, junction of wood of successive years; m, pith; ms, medullary rays; s, autumn wood. (×4.)

This difference in texture between the autumn wood and the later spring wood is quite visible to the naked eye (Fig. 44), and gives rise to a series of annual rings, each of which represents a year’s growth. Thus, a cross cut through the four year old part of a branch (Fig. 43) shows four layers of wood, one of which was formed each year. The length formed last year has two layers of wood (if we look in summer), and the current year’s growth has one layer, all of which has been formed since spring.

The advantages of secondary thickening.—The formation of secondary wood and bast is very important. The new wood is required (1) to provide additional water-vessels to supply the demands of an increasing number of leaves, and (2) to give the necessary increase of mechanical strength to the growing tree. Again, the increase in the quantity of food manufactured by the leaves makes a larger quantity of bast necessary for its distribution. Last of all, new tissues are required to take up the duties of those which are old and worn-out.

Fig. 44.—Cross section of Larch stem, showing annual rings.

In forest trees, the central pith becomes almost obliterated and the old stem practically consists of wood, bast, and bark. The various annual rings, and the bast, of a woody stem are joined together by a number of radiating horizontal spokes called medullary rays (Fig. 43, ms, ms′, ms″, ms‴). These conduct water and food materials across the stem from layer to layer. The beautiful lines and patches called “silver grain,” which may be seen in oak furniture, consist of the medullary rays exposed in radial-longitudinal section.

Bark.—The very young stem is surrounded by a thin waterproof skin, perforated by stomata (p. 53) as the skin of a leaf is; and the part immediately below the skin possesses leaf-green, and can therefore decompose carbon dioxide as a leaf does (p. 51). An old stem, on the other hand, is covered by a layer of tough bark (br, Fig. 43), which splits from time to time, owing to the stretching which is caused by the increasing thickness of the wood. The bark begins as a layer of cork, which forms on the outer side of the bast. The cork cuts off the food supply from all the external tissues, which die. Bark, therefore, consists of the cork and the dead layers outside it.

Grass stems.—The great strength of grass stems—so apparent when we try to bend a straw—is largely due to silica, the substance of which rock-crystal and ordinary sand are composed. When a straw is burnt, this remains as a hollow cylinder of mineral matter. The great strength of the cylindrical form has already been referred to. It is very well seen also in a bamboo stem, and anyone who has blunted the edge of his knife on a piece of bamboo will appreciate the additional hardness which is given by the presence of mineral matter.