2io THE ANATOMY OF WOODY PLANTS 



Fig. 153. The most significant relation of the xylem in the leaf trace 

 is by means of the series of centripetal or cryptogamic elements. 

 These are numerous and serve to bring about a very intimate and 

 copious connection between the bundle proper and the transfusion 

 tissues. There is good reason on the basis of the anatomy of 

 Prepinus and the Cordaitales to regard the transfusion tissue which 

 characterizes the foliar organization of all but the very lowest of 



the gymnosperms as 

 a product of the 

 , differentiation of the 



A centripetal or cryp- 



togamic wood. In 

 the true pines of the 

 ^^ Cretaceous the 

 centripetal wood 

 fc| was absent, as it is 

 ^jjjjr in modern species of 

 y the genus, but the 

 internal transfusion 

 sheath was often 

 we ^ developed, thus 

 showing a clear filia- 

 ti on with Prepinus. 

 FIG. 157. Leafy twig of Cosuorina equisetifolia (after ^h ere fti Q centrip- 

 Solerderer). F 



etal xylem has dis- 

 appeared, the relation between the bundle and the transfusion tissues 

 occurs on the flanks of the centrifugal wood. It will be obvious 

 to the reader that the tissues surrounding the fibrovascular bundle 

 in the conifers and their allies are of considerable interest from 

 the evolutionary standpoint. The transfusion tissue at the present 

 time has a significance in the vegetative leaves of all but the lowest 

 gymnosperms in connection with the storage of water and the con- 

 ducting of it to the green cells of the mesophyll. In the repro- 

 ductive leaves it has taken on another but equally interesting 

 function, as will be indicated in the next chapter. 



Transfusion elements are found in the leaves of all seed plants 

 from (and excluding) the cycads upward. In the conifers they are 



