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.