Cross section of a tree trunk, showing the rings of annual growth, the medullary rays, the dark heartwood, the lighter sapwood and the bark.
There are two distinct plans of branching in trees. When the main trunk extends upward to the tip, as it does in the larch and other conical trees, it is said to be excurrent, and when the main stem divides into many more or less equal divisions, as we find it in the American elm and other spreading trees, it is said to be deliquescent,—the latter form is the most common one among our deciduous trees.
TREE WITH AN EXCURRENT TRUNK
The inner structure of these dicotyledonous trunks is seen when we examine the cross section cut of a felled tree. In the centre is the heartwood, the durable wood of commercial value, the cells of which are hard and dry; next it the soft sapwood, the living part of the tree containing cells filled with sap; then the cambium layer, the zone of growing cells, and outside of this the bark. Each year new cells are formed in the cambium layer, the inner ones making new wood, the outer ones new bark, and by counting these annual rings of growth the approximate age of the tree is found. In young trees there is a conspicuous central portion of pith which remains after the tree matures, as long as the heartwood is sound. The lines radiating from the centre to the circumference are called medullary or pith rays and form the “silver grain” of the wood. As the size of the trunk increases, the bark unable to expand, cracks in fissures or peels in layers, and is pushed off by the tremendous growing power from within. The heartwood is not a living part of the structure and often trees live for years without it,—hollow shells with a normal amount of vitality so long as the roots, the cambium layer and the buds are not injured.
Branches grow from the axillary or lateral buds on the stem, continuing their growth every year by the development and unfolding of new buds, both terminal and lateral. When the growth is carried on by the terminal buds, the tree is more apt to be regular in outline than when these are injured or killed and lateral buds develop the growth instead. Branches vary in showing an upright, drooping, or horizontal habit of growth, as we see them in the Lombardy poplar, weeping willow and tupelo, and within these divisions there are other contrasts of rigidity and flexibility, with differences of color and texture as well.
Apart from the general shape of the tree, the bark on the trunk and branches is a constant help in identification. It is hard and smooth on some trees, like that of the hornbeam and beech, fissured into ridges like the sugar maple on others, it sometimes flakes off in rough plates like those of the shagbark hickory, and again in thin, brittle strips like those of the hop hornbeam, the bark peels off laterally as in the canoe birch, and occasionally becomes ridged and corky as we find it on the branches of the liquidamber and cork elm. Very often the color of the bark is distinctive as is that of the green stems of the sassafras and moosewood maple and the white, brown, pink, and yellow trunks of different birches. The taste and odor of the bark are also characteristic of certain species, as, for instance, the unpleasant, bitter taste of the black cherry, the mucilaginous taste of the slippery elm, and the aromatic fragrance of the stems of the mockernut hickory. The little dots on young bark are called lenticels, they are openings for admitting air to the inner tissues. Lenticels are conspicuous in the bark of the birches.
The presence of thorns on the trunk and branches of certain trees helps us to distinguish them from others, and the clusters of dry fruit which remain hanging on some trees through the winter are another means of identification.
Stems and Twigs