TM l-414
ARMY AIR FORCES
arrangement of cells that accounts for the great variation in the weight and strength of woods.
b. (1) The hardwoods differ from the softwoods as a class in the presence of larger cells, constituting pores, scattered among the smaller ones which are mostly fibers. In many hardwoods the pores may be seen distinctly without, magnification, as small holes on smoothly cut cross sections and as fine grooves on planed longitudinal surfaces.
(2) In softwoods, the bulk of the wood is composed of fibrous cells which may be seen on a smoothly cut magnified cross section.
5. Medullary rays, also known as pith rays, are rows of cells which extend radially from the center of the tree (see figs. 1 and 2).
They vary greatly in size, being so small in many woods as to be invisible without magnification. In oak, however, they are very large and account for the large silvery patches so characteristic of this wood when it is quarter-sawed.
FIGURE 3.-Magnified view of annual ring.
6. Springwood and summerwood.-Each annual ring (fig. 3) is composed of two regions or parts : the springwood and the summer-wood. Springwood is the wood formed on the inner side of the annual ring during the early part of the growing season. It is usually more porous, softer, weaker, and especially in conifers, lighter in color than the summerwood, which is formed in the outer part of the annual ring