essels, those of the spring-wood in oak
being the most conspicuous (see Fig. 5). So that in an oak table, the
darker, shaded parts are the spring-wood, the lighter unicolored parts
the summer-wood. On closer examination of the smooth cross-section of
oak, the spring-wood part of the ring is found to be formed in great
part of pores; large, round, or oval openings made by the cut through
long vessels. These are separated by a grayish and quite porous
tissue (see Fig. 6, A), which continues here and there in the form of
radial, often branched, patches (not the pith rays) into and through
the summer-wood to the spring-wood of the next ring. The large vessels
of the spring-wood, occupying six to ten per cent of the volume of a
log in very good oak, and twenty-five per cent or more in inferior and
narrow-ringed timber, are a very important feature, since it is
evident that the greater their share in the volume, the lighter and
weaker the wood. They are smallest near the pith, and grow wider
outward. They are wider in the stem than limb, and seem to be of
indefinite length, forming open channels, in some cases probably as
long as the tree itself. Scattered through the radiating gray patches
of porous wood are vessels similar to those of the spring-wood, but
decidedly smaller. These vessels are usually fewer and larger near the
outer portions of the ring. Their number and size can be utilized to
distinguish the oaks classed as white oaks from those classed as black
and red oaks. They are fewer and larger in red oaks, smaller but much
more numerous in white oaks. The summer-wood, except for these radial,
grayish patches, is dark colored and firm. This firm portion, divided
into bodies or strands by these patches of porous wood, and also by
fine, wavy, concentric lines of short, thin-walled cells (see Fig. 6,
A), consists of thin-walled fibres (see Fig. 7, B), and is the chief
element of strength in oak wood. In good white oak it forms one-half
or more of the wood, if it cuts like horn, and the cut surface is
shiny, and of a deep chocolate brown color. In very narrow-ringed wood
and in inferior red oak it is usually much reduced in quantity as well
as quality. The pith rays of the oak, unlike those of the coniferous
woods, are at least in part very large and conspicuous. (See Fig. 4;
their height indicated by the letter _a_, and their width by the
letter _b_.) The large medullary rays of oak are often twenty and more
cells wide,