l68 MORPHOLOGY OF GYMNOSPERMS 



is usually referred to Cordaitales, but they differ from other known 

 stems of Cordaitales in certain important features. Pitys is known 

 as yet only from its pith and wood, the latter being araucarian (and 

 therefore cordaitean) in t}rpe, but with remarkably broad medullary 

 rays. The principal feature, however, is the occurrence of numerous 

 small mesarch strands which are imbedded in the pith at some distance 

 from the cylinder of wood, and which must represent the primary 

 xylem. The araucarian cylinder of secondary wood and the mesarch 

 primary cylinder is a combination of features belonging to Cordaitales 

 and Lyginodendron. Dadoxylon Spenceri, probably from the Upper 

 Carboniferous, also has mesarch primary wood, cordaitean secondary 

 wood, and double leaf traces. Scott (9) says that this stem "sug- 

 gests, perhaps more strongly than any of the other species described, 

 a truly gymnospermous stem, which may well have belonged to one 

 of the Cordaiteae, but which still retains the last relics of the primary 

 wood structure characteristic of the Poroxyleae and Lyginodendreae." 

 He is of the opinion (25) that Pitys and Dadoxylon probably belong 

 to a plexus of forms connecting Cycadofilicales of the Lyginodendron 

 type with Cordaites, but nearer to the latter. Therefore, he proposes 

 to recognize Cordaitales as including the three groups Poroxyleae, 

 Pityeae, and Cordaiteae, in which there are all gradations from 

 primary wood that is distinctly mesarch to that which is completely 

 endarch. In our terminology these three names would be Poroxy- 

 lineae, Pityneae, and Cordaitineae. 



THE LEAF 



The leaves are borne in spiral succession on ultimate branches. 

 They are usually simple, elongated, with parallel and repeatedly 

 dichotomous venation (except in the grassUke forms). Naturally 

 they were first thought to be the leaves of monocotyledons. They 

 occur sometimes in dense masses, packed together in layers "like 

 damp beech leaves on the ground of our forests" (5). 



There is considerable diversity of form, which has been made the 

 basis for generic distinctions. For example, the leaves of Cordaites 

 are spatulate with blunt ends (fig. 199), reaching a meter in length 

 and about 15 cm. in width; those of Dorycordaites are lanceolate and 

 sharp pointed (fig. 195), and little inferior in length; those of Poacor- 



