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 - 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 



