The path of the food made by the leaves.—The plant food which the leaves make is drained off into the stem, and distributed to the parts where growth is taking place. This food travels in the soft tissue—called the bast—which lies below the bark. This fact can be shown indirectly by removing a ring of bast from the lower part of a branch, say of willow, and putting the branch into water. When at length the cutting puts out roots, these spring from the top of the ring. If any spring from the stripped part they are markedly smaller, and fewer in number. In an uninjured cutting, which is of course supplied with prepared food along all its length by the bast vessels, such roots spring from the cut end. The supply of leaf-made food can also be cut off by ligaturing a twig below the leaves, as by twisting a wire or cord tightly round it. In such a case growth usually ceases in the part of the twig below the strangled part, while the upper part of the twig, to which the leaf-made food is now restricted, grows much more luxuriantly than before. Gardeners often produce unusually fine fruits by ligaturing the lower parts of the twigs on which the fruits are ripening. 12. HOW STEMS ARE STRENGTHENED. 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. [Pg 72] 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?