228 



CORDAITBAE 



[CH. 



is a narrower sheath of thick-walled cells more sharply contrasted 

 with the mesophyll. The vascular bundles agree in structure 

 with those in the rachis of a Cycadean frond more closely than 

 with the veins of an Araucarian or other Coniferous leaf. The 

 xylem consists mainly of centripetal elements which form a 

 deltoid strand with the protoxylem at the apex, and in close 

 association with this is a larger or smaller amount of narrower 

 centrifugal tracheids : in the section shown in fig. 465 the centri- 

 fugal xylem may extend all round the centripetal tracheids, but 

 it usually forms an irregular arch with its base attached to the 

 sides of the larger tracheal strand, cp, separated, except at the 



Fig. 465. Cordaiies leaf {Cordaites Felicia Bens.), probably borne on a MesoxyUm 

 stem, cp, eentripetal xylem ; ph. phloem ; px, protoxylem. (Kidston CoU. 

 No. 2194.) 



base of the arch, by a small amount of conjunctive parenchyma 

 from the centripetal xylem. The phloem is not preserved and is 

 represented only by a few patches, ph, below the centrifugal 

 tracheids. Dr Benson^ in her account of this type of leaf gives 

 additional details and compares the anatomical features with 

 those in other species. The dual nature of the xylem like that 

 characteristic of recent Cycads has usually been regarded as a 

 definite feature of Cordaites leaves ; Dr Stopes, on the other hand, 



' Benson (12). 



