XLIV] PITYOXYLON 223 



small group in the centre of a clear crystalline matrix. Some of 

 these patches simulate resin-canals, a fact which led Schenk to 

 deny the existence of true canals. All stages of decay are shown, 

 from the partial obliteration of a circular group of tracheids to 

 the destruction of the group, one central element being left, or to 

 the formation of a canal-like cavity (fig. 725, A). It is often 

 difficult to decide whether a clear space in the wood is a canal or 

 the result of post-mortem changes, but there is no doubt as to the 

 occurrence of some true secretory canals in different regions of the 

 wood. There is very little parenchyma accompanying the canals. 

 The medullary rays are of two kinds, uniseriate, 1 13 cells in 

 depth, though usually about 6 cells in depth, and lenticular rays 

 with a central canal (fig. 725. C, c) identical with those in a modern 

 Pine. Fig. 725, D shows part of a vertical canal with some 

 parenchymatous lining in continuity with a hoiizontal canal in a 

 broad medullary ray. Several small pits occur on the tangential 

 and horizontal walls of the ray cells, and the radial walls, which 

 are less clearly preserved, occasionally show 1- 3 elliptical pits. 

 In radial section the upper and lower cells of a medullary ray are 

 often distinguished by their less uniform breadth and resemble 

 in this respect ray tracheids. A careful examination of sections 

 revealed the existence of bordered pits in the tangential walls of 

 these elements and confirmed their tracheal nature 1 . 



The bordered pits are generally single and sparsely scattered on 

 the radial walls of the tracheids; occasionally the pits are in 

 contact and a few double and opposite rows occur. An interesting 

 feature is the occurrence in some tracheids of a biconcave patch 

 of some brown substance agreeing closely with resinous deposits 

 described by Penhallow 2 . If this species is from Jurassic strata 

 its close resemblance to recent types of Pinus is a fact of consider- 

 able interest. 



Pityoxylon Ruffordi Seward. 



This species 3 is founded on a specimen of wood obtained by 

 Mr Rufford from Wealden beds at Ecclesbourne on the Sussex 



1 I am indebted to Miss Ruth Holden for calling my attention to this feature 

 and for other assistance in the examination of. the sections. 



2 Penhallow (04) p. 526. 



3 Seward (95) A. p. 199; (96) p. 417, Pis. ii., in. 



