THE LEAF 



205 



elongated and thicker-walled inner sheath being absent. The 

 intimate connection between the centripetal or cryptogamic wood 

 and the transfusion tissue is observed in all cases. It is clear 

 accordingly in the Cordaitales that the transfusion tissue which, as 

 will be subsequently shown, plays an interesting role in the evolu- 

 tionary history of higher gymnospermous groups is primarily related 

 to the centripetal or cryptogamic wood. 



FIG. 152. Longitudinal section of a leaf bundle in Cordaites principalis 



The organization of the leaf in the conifers will next occupy our 

 attention, since the present chapter deals with the leaf only in the 

 features which are of general evolutionary importance. Fig. 153 

 shows an enlarged view of the whole leaf of Pinus strobus, the white 

 pine. The epidermis, reinforced by a hypoderma, surrounds the 

 median green part of the leaf, the mesophyll, in which lie resin canals 

 and a single fibrovascular bundle. The latter, exceptionally for the 

 conifers, is marked off from the surrounding fundamental tissues 

 by a well-defined circular endodermis. The fibrovascular strand 

 consists of xylem entirely centrifugal and mostly secondary in its 

 origin, and this meets with the phloem on its lower border. Sur- 

 rounding the conducting strand of the leaf is a mixture of ordinary 



