CONIFERALES 



333 



elements are arranged in radial rows and usually increase in size 

 toward the upper surface of the leaf. In the xylem directed down- 

 ward are seen indications of wood rays, and the inspection of the 

 longitudinal aspect reveals the fact that it is made up largely of 

 tracheids with bordered pits. This is the centrifugal xylem and 

 corresponds to the mass of wood in the foliar trace of the fascicular 



FIG. 241. Portion of leaf of Prepimis statenensis 



leaves of living pines. The upwardly developing wood is the 

 cryptogamic xylem and confirms the conclusion as to the affinity 

 of Prepinus, already suggested by the organization of its trans- 

 fusion sheath, namely, that the genus is allied to the Cordaitales. 

 It is thus apparent that the details of organization of the leaf in 

 Prepinus, which in turn is clearly the ancestor of Pinus, justify 

 an attribution of cordaitean ancestry to the Abietineae. This 

 conclusion as to relationship is supported by the primitively cor- 

 daitean character of the pitting which so strikingly indicates a 



