THE LEAF 



207 



A link between the conditions found in the case of Pinus and 

 the anatomical organization of the Cordaitales is presented by a 

 mesozoic fossil form allied to Pinus to which the name Prepinus has 

 been given. Fig. 154 illustrates the organization of the leaf in this 

 species. It is bounded by flat surfaces as a result of contact with 

 surrounding leaves of the many-leaved fascicle. The hypodermal 

 structures are strongly developed as in the Cordaitales and con- 

 stitute the same strengthening ribs as are found in the leaves of 



FIG. 154. Transverse section of leaf of Prepinus slalencnsis 



that genus. On account of the changes resulting from fossiliza- 

 tion the endodermis is not so distinct as in living pines, and the 

 mesophyll or soft substance of the leaf is very poorly developed. 

 The bundle is surrounded by a very dense transfusion sheath in 

 which all the cells are empty and provided with bordered pits. 

 Not only is the transfusion sheath massive in Prepinus and entirely 

 composed of transfusion tracheids to the exclusion of paren- 

 chymatous elements, but it is further complicated by the presence 

 of an internal thick-walled zone comparable with the similar 

 structure in C. principalis figured above. In the comparatively 



