Page:The Oak.djvu/156

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138
THE OAK

twelve per mm.) fine ones between them, which undulate between the vessels. In slowly-grown close wood there is no vestige of radial arrangement left.

In the tangential section the small medullary rays are seen to consist each of a vertical row of a few cells, the large ones having numerous cells (see Fig. 27).

Wood-parenchyma cells broader than small medullary rays, and the color is chiefly due to pigment in these wood- and ray-cells. The wood-cells are pitted with oblique, slit-shaped, simple pits.

The vessels have bordered pits, and the septa are perforated each by one large circular opening. The smaller vessels have delicate spirals on their walls as well as bordered pits.

Nördlinger says that pith-flecks occur occasionally.

It is impossible to distinguish between the wood of the varieties pedunculata and sessiliflora.

(2) Its density varies considerably. Taking the weight of a given volume of water as unity, the weight of an equal volume of oak timber may weigh from 0·633 when air-dry to 1·280 when fresh cut. We may take the average density of green—i.e., newly-felled—oak with all its sap present, as about 1·075, and that of the seasoned wood as about 0·78.

It must be borne in mind, however, that these weights refer to the wood as a structure—that is, a complex of vessels and cells, etc., containing air and liquids—and do not give the specific gravity of the wood substance itself. The latter may be obtained by driving off all the air and