740. Opening of the buds.—When the young shoot begins to grow in the spring, the bud-scales are forced apart or open of their own accord. During the young condition the shoot is very soft and brittle, and also possesses a very thin, little cutinized epidermis. In this condition it is especially liable to mechanical injury and to injury from drying out. We find, therefore, a tendency for the inner bud-scales to elongate during vernation, thus forming a tube around the delicate tissue much like the opening out of a telescope. The young leaves and internodes themselves are often provided with a woody or hairy covering to retard transpiration. When the epidermis becomes more efficient the hairy covering often falls away.
In the case of naked buds protection is afforded in other ways: by the protection of hairy covering, by physiological adaptation of the tissue, or in many cases by the late appearance of the shoot in spring after the very dry April and May winds have ceased.
741. Bud-scars, and how to tell the age of the plant.—In general the bud-scales when they fall away in the spring leave scars termed scale-scars, and the whole aggregate of scale-scars makes up the bud-scar. The position of the buds of previous winters is, therefore, marked. It becomes an easy matter to determine the age of a branch, since all that is necessary is to follow back from one bud-scar to another, the portion of the stem between representing, except in rare cases, one year’s growth.
A woody plant grows in height only by the formation of new sections of stem added to the apex or side of similar sections produced the previous season, never, as is commonly supposed, by the further elongation of the previous year’s growth. Hence a branch once formed upon a tree is fixed as regards its distance from the ground. The apparent rise of the branches away from the ground in forest trees is an illusion caused by the dying away of the lower branches.
742. Definite and indefinite growth.—With the opening of the buds in spring, growth begins. In some cases, when all the members for the season were formed, but still minute, within the bud, such growth consists solely in the expansion of parts already formed; in others only a few members are thus present to expand, while new ones are produced by the growing point as the season progresses. In most cases growth is completed by the middle of July, soon after which buds are formed for next year’s growth. Such a method of growth is termed definite.
In a few woody plants, as, for example, sumach, locust, and raspberry, growth continues until late in the autumn. In such cases the most recently formed nodes and internodes are unable to become sufficiently “hardened” before winter sets in, and are killed back more or less. Next season’s shoot is a branch from some axillary bud. Such growth is termed indefinite.
Fig. 432.
Three-year-old twig of the American ash,
with sections of each year’s growth
showing annual rings.
743. Structure of woody stems.—If we make a cross-section of a woody twig three general regions are presented to view. On the outside is the rather soft, often greenish “bark,” so called, made up of sieve tubes, ordinary parenchyma cells, and in many cases long fibrous cells composing the “fibrous bark.” From a growing layer in this region, termed the phellogen, the true corky bark of the older trunk is formed.
Next within the bark we find the so-called “woody” portion of the twig. This is strong and resistant to both breaking and cutting. The microscope shows it to be composed of the ordinary already known woody elements,[43] wood fibers, for strengthening purposes, pitted and spiral vessels as conducting tissue; and intermixed with these some living parenchyma cells. A cross-section of the stem also shows narrow radial lines through the wood. These are pith-rays, composed of vertical plates of living parenchyma cells. These cells, unlike the others in the wood, are elongated radially, not vertically. The height of the pith-rays as well as their thickness varies with the species studied. In the older trunk only the outer portion, a few inches in thickness, remains light-colored and fresh, and is called sap-wood. The inner wood is usually darker and harder, and is termed heart-wood. Living parenchyma cells, in general, are present only in the sap-wood, and in this almost solely the ascent of sap occurs. Dyestuffs and other substances are frequently deposited in the walls of the heart-wood.