300 THE ANATOMY OF WOODY PLANTS 



discovered to be exemplified in a remarkable manner by the 

 observation of the wood in the axes of members of the Conifer- 

 ales, as will be noted at a later stage. 



The leaves of the Cycadales are of great importance from the 

 standpoint of evolutionary anatomy. Here the centripetal wood, 

 which is poorly developed at best in reproductive axes, is abundant, 

 well defined, and universal in its occurrence. Fig. 213 illustrates 

 the transverse section of a leaflet of Cycas revoluta. It is apparent 



FIG. 213. Transverse section of a foliar strand in Cycas revoluta 



that the mass of the xylem is of primary origin and spreads out 

 in a fanlike fashion toward the upper surface of the leaf. It is 

 therefore centripetal in its development. Below the mass of 

 centripetal xylem lies a much more scanty development in the 

 region of the regularly arranged phloem. This is the centrifugal 

 xylem, which is partially primary and partially secondary in its 

 origin. Fig. 214 illustrates the longitudinal section of a leaf 

 trace in Cycas. The phloem, easily recognized by its characteristic 

 sieve tubes, is seen toward the right. To the left is situated the 

 centripetal wood, composed from left to right of pitted, scalariform, 

 reticulate, and spiral elements, the latter constituting the protoxy- 



