332 THE ANATOMY OF WOODY PLANTS 



the genus. Within the angular outline of the leaf in Prepinus 

 is seen, beneath the epidermis, the ribbed hypodermal tissues, 

 recalling those of the older gymnosperms. The cortical region 

 of the leaf terminates in a not very clearly marked endodermis 

 which doubtless owes its loss of denniteness to the changes resulting 

 from fossilization. Within the endodermal boundary lies the 

 entirely tracheary and strongly pitted transfusion tissue of the 



FIG. 240. Leaf of Prepinus slatenensis 



leaf. The elements of this category are differentiated into two 

 zones an outer one composed of short, broad, pitted elements, 

 and an inner one consisting of thick-walled cells of narrow lumen. 

 The former are known as the outer transfusion sheath and the 

 latter as the inner transfusion sheath. Both structures have 

 their counterpart in the leaf of certain of the Cordaitales. The 

 double transfusion sheath was also frequently present in the foliar 

 organs of true pines of the American Cretaceous. Another most 

 interesting feature of the organization of Prepinus was the struc- 

 ture of the xylem. As may be seen from Fig. 241, the wood 

 presents two regions, an upper and a lower. In the former the 



