THE IDENTIFICATION of woody plants in winter has a well-developed technique of its own which, while quite different from the techniques used for the determination of the same plants during the other seasons of the year, may be fully as reliable. Most woody plants have an individuality of their own which is just as evident in winter, at least upon careful examination, as it is in summer. The morphological features used are, of course, not usually those of flowers or fruits, or even of leaves, but the size and form of the plants and especially the structures of the younger branches, or twigs.
Size and Form of Woody Plants. The sizes of woody plants are, of course, quite variable, but an effort is made, in the descriptions of the following pages, to give the general range of the height (and for trees, especially, the trunk diameter) of mature plants. It is realized that trees, when young, are no taller than shrubs, and it may be difficult in winter to tell if a given plant has reached maturity or not, but the figures would be meaningless otherwise.
Trees and shrubs grown in the open tend to have a characteristic form well illustrated by such species as white pine, Lombardy poplar, weeping willow, and American elm. In general, however, under forest conditions the crown is restricted in its development by the competition of surrounding trees and the form is not so distinctive.
Bark. Bark, or periderm, is that portion of a woody stem which lies outside of the cambium layer, and is protective in nature. The very youngest stems are covered by epidermis, but this is usually quickly succeeded by periderm, having its origin in superficial layers of the cortex. Compared with older bark, this is quite smooth, broken only by lenticels. At this stage, color is an important feature for diagnostic purposes. On older branches or trunks cork cambium (Fig. 1) originates deeper in the stem and when these deeper layers are formed the tissues on the outside die and the bark tends to become scaly or furrowed (Fig. 4), often according a very distinctive pattern. In sour gum, flowering dogwood, and persimmon, for example, the bark is divided into small squarish plates and has the general appearance of alligator leather (Fig. 5). In cherries and birches the bark peels horizontally into thin sheets (Fig. 3). Broad exfoliating plates characterize certain hickories. Deeply furrowed bark is found on many trees, as chestnut oak (Fig. 2). Lumbermen often rely almost wholly upon bark characteristics in the identification of timber species, but the features are often difficult to describe so they can be recognized. Experience is most important in this type of identification.
Fig. I GENERALIZED SECTION OF WOODY STEM, SHOWING ORIGIN OF CORK CAMBIUM.
Fig. 2 BARK OF CHESTNUT OAK.
Fig. 3 BARK OF YELLOW BIRCH.
Fig 4 BARK OF TULIPTREE.
Fig. 5 BARK OF SOUR GUM.