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 medullary 

 rays, and the color is chiefly due to pigment in these 

 wood- and ray-cells. The wood-cells are pitted with ob- 

 lique, slit-shaped, simple pits. 



The vessels have bordered pits, and the septa are per- 

 forated each by one large circular opening. The smaller 

 vessels have delicate spirals on their walls as well as bor- 

 dered pits. 



Nordlinger says that pith-flecks occur occasionally. 



It is impossible to distinguish between the \vood 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 T280 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 sea- 

 soned 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 



