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 [Pg 74] 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), [Pg 75] 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. 13. CLIMBING STEMS. 1. Hooking stems.—Examine a bramble or a wild rose plant in a hedge. Why does it need to climb? How does it climb? Pull a branch and notice by what means it resists the pull. What is the shape of the prickles? Do they point upwards or downwards? On what parts of the plant are the prickles found? Notice how easily a prickle may be pushed off, sideways. Is it as easy to tear it off lengthways? Is the prickle a little branch, or merely an extension of the rind?