xxxiv] 



PITYS 



287 



phloem, is raised by the fact that the primary strands are composed 

 exclusively of xylem and are in most cases separated from the 

 secondary wood by several parenchymatous cells. Scott favours 

 the view that the primary phloem was originally at some distance 

 from the primary xylem, the cambium being formed nearer to 

 the phloem, an arrangement foreign to recent Gymnosperms. 

 It is perhaps conceivable that the primary conducting strands 

 included no true phloem. 



Fig. 487. Pitya antiqua. A, radial section of secondary xyiem; B, transverse 

 section showing the inner edge of the secondary wood and two primary-xylem 

 strands with protoxylem, poc. (Kidston Coll.) 



The secondary xylem consists of tracheids with 4 — 5 alternate 

 rows of hexagonal pits on the radial walls (fig. 487, A) and not 

 infrequently on the tangential faces. Near the ends of the 

 tracheids the pits are occasionally more scattered and separate 

 and may be reduced to a single row^, but on the whole the pitting 

 is essentially Araucarian. The medullary rays are generally 

 4 cells in breadth, but may be 7 cells broad. In depth the rays 

 extend to 70 cells. As seen in fig. 487, B, the inner ends of the 

 rays are especially broad owing to the tangential dilatation of 

 the cells. The innermost secondary tracheids are characterised 

 by pitting intermediate between spiral and reticulate. There 

 are no regular rings of growth in the wood. 



Pitys frimaeva Witham. This species^, also founded on 

 material from the Calciferous sandstone of Berwickshire, differs 



1 Witham (33) A. PI. vin. fig. 2. 



2 Ibid. p. 39, Pis. vm., xvi. ; Scott (02) p. 335, Pis. ir., vi. 



