Among the wood-cells are found short cells, wood-parenchyma, that remain alive long after the other cells are dead. One of their chief functions is to store starch and other foods that are conveyed to them by the medullary rays or silver grain. These consist of elongated cells that run at right angles to the course of the wood-cells ; they serve to convey gases as well as food. Much elaborated food, espe cially proteids, is conveyed by the bast. Most pro teids are unable to pass through cell-walls and so are able to move only in the large cells, or sieve tubes of the bast, whose end walls or sieve plates are pierced with holes. The bast contains smaller cells known as companion cells and bast parenchyma which remain alive after the sieve cells are apparently dead ; their function is not clearly understood.
In dicotyledonous plants, between the wood and the bast is found the cambium, an embryonic tissue that forms new cells whose growth causes the stem to thicken year by year. The inner part of this growth becomes wood, which adds an "annual ring." These rings are clearly marked, because the wood formed in the fall is denser and has smaller cells than that formed in the spring. The outer part of the new growth becomes bast, which wears away on the outside almost as fast as it forms within, and. in consequence, does not thicken much from year to year. Monocotyledonous stems have no cambium and do not grow thicker from year to year.
The cambium causes the cion to unite to the stock ; it heals wounds, such as are made by pruning, by forming a tissue called callus. This sometimes produces new buds, whose growth com pensates for the part cut away. At the tip of the stem the cambium does not form a complete ring but is confined to the fibro-vascular bundles. In trees and shrubs it gradually extends itself from one bundle to another, thus forming a complete ring. As soon as this is accomplished, it begins to form a complete ring of wood within and of bast without. In herbs no such complete ring is formed.
Outside the bast is found the rind or cortex, which is usually green, and, in consequence, manu factures starch. It also serves to convey starch. This is easily seen when it is cut away all around the tree, in the process of " ringing," whereupon the tissues below lose their starch. If the bast be cut through also, the supply of proteids is cut off and death soon ensues. As the stem grows older, layers of cork are formed in the rind. These cut off the tis
sues lying outside them, which soon die and so form bark. The very first layers of cork are formed on the extreme outside of the stem, and are interrupted at frequent intervals by breathing pores or lenti eels.
At the very tip of the stem is found embryonic tissue which continually forms new cells ; this is called the growing point. Just back of this, new ]eaves arise as protuber ances (Fig. 38). These rapidly grow larger and fold over in such a way as to protect the growing point from mechanical in juries as well as from dry ing. The various waxes, resins and furry coverings of buds are not for protection against cold, as popu larly supposed, but for protection against drying.
The crowding of the young leaves at the grow ing point, forces them to take on a regular ar rangement which largely determines the arrange ment of the branches, since these arise from the point (axil) where the leaf is joined to the stem, Not all branches develop. Many that start cannot get sufficient light and soon die. This is known as " self-pruning," and is seen especially in forest trees, which produce lumber free from knots. Many buds do not even start to de velop but remain dormant, often for many years, growing just enough to keep pace with the annual thickening of the tree.
They may be traced back to the center of the tree, sometimes several feet long, but no thicker than a lead-pencil. New or adven titious buds may be formed; such buds, becoming crowded and distorting the grain of the wood, cause the appearance familiar in bird's-eye maple.
The stem requires strengthening tissue in order to sustain the weight of its branches and the force of the wind. In the tree the accumulated wood serves every purpose, but in the herbaceous stem special strengthening tissue is formed, quite dis tinct from the wood. In parts of the stem that are lengthening, this tissue consists of collenchyma cells, whose walls, thickened at the corners only, have thin places by means of which food and water may be absorbed (Fig. 24). Their growth keeps pace with that of the stem, otherwise they would soon break and become useless. In older parts of the stem that have ceased lengthening, the mechanical cells, sclerenchyma, have walls equally thickened all around, except at the pits ; when the thickening reaches a certain point the cells die, but their use fulness is not impaired thereby.