138 THE OAK 
twelve per mm.) fine ones between them, which undu- 
late 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 medul- 
lary rays, and the colour 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. 
Nordlinger 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 com- 
plex of vessels and cells, &c., containing air and liquids 
—and do not give the specific gravity of the wood sub- 
stance itself. The latter may be obtained by driving 
