Pith Rays In addition to the cells whose length is parallel to the trunk of the tree, wood also contains other layers of cells of a different character whose length is at right angles to the trunk of the tree. These cells occur in thin sheets radiating from the bark toward the pith, and form what are called the "pith rays" or "medullary rays" of wood. They are best seen on a quartered section, and are what gives the beautiful, flaky appearance to quartered oak and sycamore. The pith rays are less conspicuous in beech, maple, and birch, and are scarcely or not at all visible to the naked eye in the pines and many other woods.
Springwood and Summerwood When growth begins in the spring, the new cells are large and thin-walled. As the season progresses, smaller and thicker-walled cells are produced, until the last growth of the summer is much denser than the spring growth. It is this contrast between early spring and late summer wood that enables us to distinguish the rings of yearly growth upon a stump or crosssection of a piece of timber. The transition from the large,' thin-walled cells of spring to the small, thick-walled cells of summer may be abrupt, as in the yellow pines, or very gradual, as in the white pines and the firs. In the former, the bands of dense wood are very conspicuous; in the latter, they are sometimes scarcely visible to the naked eye. Counting these annual rings on the stump affords an easy and practically accurate means of determining the age of our common trees. Trees which grow in warm climates where there are no fixed cycles of growth and inactivity, do not develop annual rings.
Among the softest, most easily worked woods are white pine, spruce, basswood, and yellow poplar. The first two are non-porous; the last two, diffuse-porous. In all, the transition from springwood to summerwood is very gradual; the cells are thin-walled; and the texture is remarkably uniform. None of these woods, however, has great strength. Hickory and osage orange, two of our strongest native woods, contain such large pores that, at first glance, one might think they were not strong; but closer examination under the microscope shows a multitude of very small, thick-walled cells which are the source of their remarkable strength.
Sapwood and Heartwood A cross-section of the trunk of a living tree will show on the outside a belt of wood of varying width, in which the vital processes of the tree are carried on. Within this belt is a cylinder of older cells, no longer of importance in the growth of the tree, whose function is chiefly that of a support for the great weight of the crown. The outer belt is called the "sapwood;" and the inner cylinder, the "heartwood." The sapwood is light-colored. When tapped, sap flows from it, as in the maples; or resin, as in the pines. As the cells become older, their functions are assumed by newer ones closer to the bark. The living matter of the older cells is gradually changed by deposits of mineral or other matter, generally of darker color, which produce what is called "heartwood." It is the dark, richly colored heart of birch, red gum; black walnut, red cedar, redwood, dogwood, persimmon, and other trees that yields the beautiful woods for which these species are noted.
Heartwood develops very early in some species, like black locust, osage orange, and catalpa, and very slowly in other species. Black walnut is likely to reach an age of fifty years before much dark heartwood—the valuable portion of the tree—is formed.
The heartwood in some species—basswood and hemlock, for example—is often not clearly distinguishable from the sapwood, and the older cells seem to retain the ability to transmit sap. That the outer portion of the trunk is the main seat of vital activity, however, is proved by the continued growth of trees for many years after they become hollow at the base through decay.
Heartwood is generally heavier than sapwood, and fully as strong if equally free from defects. Moreover, it is usually much more resistant to decay. On the other hand, since its cells are more open, sapwood usually absorbs wood preservatives better than heartwood.
The Figure of Wood The varying combinations of cells of different kinds, of springwood and summerwood, of heartwood and sapwood, of slow and rapid growth, of knots, burls, dormant buds, and spiral or "curly" grain, produce the many beautiful and characteristic figures which give wood a unique position as a decorative material. These natural variations are still more accentuated by methods of sawing and working, so that the artificer of wood can produce an endless variety of effects without monotony.
Weight and Strength Other factors being equal, the strength of wood is roughly proportional to the dry weight. Hence heavy, thick-walled cells are stronger than light, thin-walled cells; and summerwood stronger than springwood. Given two pieces of wood of the same kind and equally free from defects, the one which is the heavier and con tains the larger proportion of summerwood is the stronger. This affords a ready and fairly accurate means of selecting certain kinds of timber. Comparisons of the weight and strength of a number of woods are given on page 19.
What the Microscope Shows Cross-sections of four common woods, magnified to the same degree, are shown in the illustrations. Since the magnification is the same throughout, the character and size of the cells in these woods are readily compared. Balsam fir and longleaf pine are non-porous woods; birch, diffuse-porous; and oak, ring-porous. In longleaf pine, the transition from spring to summer wood is abrupt, resulting in alternating light and dark bands. In the other woods, the transition is very gradual, and often not conspicuous to the naked eye. Comparing size and thickness of cell walls, it is seen that, for the entire season's growth, the cells of balsam average the largest and thinnest-walled; those of longleaf pine rank next; those of birch next; and that the oak cells are the smallest and thickest-walled. The ragged openings in the longleaf pine are not pores; they are ducts in which the resin forms.