186 CYCADOXYLEAE [CH. 



may reach a considerable breadth, of secondary centrifugally 

 developed xylem and phloem enclosing a large pith containing 

 either a narrow, peripherally placed, and more or less continuous 

 cylinder of inversely orientated conducting tissue or scattered 

 bands of centripetal xylem and phloem. The secondary xylem 

 is manoxylic, while the internal vascular tissue recalls that of 

 Ptychoxylon and to a less extent the inverted arcs that are rarely 

 met with in Lyginopteris stems. A brief diagnosis of two species 

 may serve to illustrate the genus : a third species is included in 

 Cycadoxylon, but it is founded on material too incomplete to 

 admit of satisfactory diagnosis. 



Cycadoxylon Fremyi Renault. 



This Permian speciesMs represented by a pieceof stem 2 2-5cm. 

 in diameter (fig. 453, B) characterised by (i) a fairly broad 

 parenchymatous cortex with secretory canals and several hypo- 

 dermal nests of sclerous tissue, (ii) a cylinder of secondary xylem 

 and phloem nearly the whole of which is centrifugal, (iii) a large 

 pith containing several scattered narrower bands or arcs of 

 centripetally developed xylem and phloem. The tracheids, with 

 4 6 series of hexagonal pits, form radially disposed rows, 1 4 

 elements broad, separated by broad and deep medullary rays, 

 Renault does not mention the occurrence of any primary xylem 

 as distinct from the secondary centrifugal xylem, but in a section 

 which I examined some years ago in his laboratory there 

 appeared to be a group of primary tracheids. There are no 

 anastomoses between the main cylinder and the internal bands 

 of inversely orientated tissue. 



Cycadoxylon robustum (Seward). 



This species 2 is based on a piece of stem from the Lower Coal 

 Measures of Lancashire first described by Williamson and identi- 

 fied as an unusually large example of Lyginopteris. Williamson 

 and Scott, while recognising certain features in addition to the 

 large size of the stem, which must have reached 14 cm. in diameter, 

 expressed the view that 'there is a presumption that it really 

 belonged to a Lyginodendron, or to some plant of the same type 

 of structure.' The examination of additional material led me to 

 1 Renault (96) A. p. 308, figs. 55, 56. 2 Seward (97 2 ). 



