CORDAITALES 



167 



petiole. If the cordaitean leaves of Poroxylon (see below) are added 

 to this combii^tion of the vascular anatomy of Lyginodendron and 

 Cordaites, there seems to be good ground for regarding Poroxylon 

 as one of the Cordaitales with strong Lyginodendron affinities; the 

 foliage will hardly permit the converse alternative. Certain stems 

 recently discovered in the Lower Coal-measures of England were 

 referred at first to Poroxylon (25), but these, together with certain 

 other stems referred to Cordaites, have been made by Scott and 

 Maslen (27) the basis of the new genus Mesoxylon, whose name 



¥m- ■■■■ ■'. '■■ * •/■ .■■"hM\ 



Fig. 198. — Cordaites sp.: part of transverse section of stem; p, pith; 

 pliloem; //, leaf trace; c, cortex; X6 — After Scott (25). 



X, xylem; 



suggests its intermediate character. The pith is relatively large 

 and discoid (as in Cordaites); the leaf traces are double at first, but 

 divide further before entering the leaf; and the leaf traces are mesarch 

 from the first (as in Poroxylon) . The genus is regarded by its authors 

 " to completely bridge the gap, so far as anatomy is concerned, between 

 the Poroxyleae and the Cordaiteae." 



From the Lower Carboniferous of Scotland, Scott (9) has obtained 

 evidence of stems that show a connection between Cycadofilicales 

 and Cordaitales even more clearly than does Poroxylon, and they 

 have the advantage of being in a far better historical position to play 

 the r61e of actual connecting forms. The group of stems considered 



