CORDAITALES AND GINKGQALES 



307 



elements of the secondary wood. This is well shown in Fig. 220. 



Among the living conifers this condition is most nearly paralleled 



by the Abietineae. 

 The foliar trace 



is of course of great 



importance in this 



as in other ancient 



gymnosperms. 



Fig. 221 illustrates 



the organization of 



one of the bundles 



of a broad cordai- 



tean leaf (Cordaites 



principalis) in both 



transverse and 



longitudinal section. 



In a is shown the 



transverse topog- 



raphy of the strand, 



and it is clear that the centripetal wood is well developed, ending 



upwardly in large 

 elements which are 

 pitted in their char- 

 acter. In the par- 

 ticular instance 

 figured there hap- 

 pens to be no devel- 

 opment of the 

 centrifugal xylem, so 

 that the phloem 

 abuts immediately 

 on the protoxylem. 

 With the flanks of 

 the metaxylem is 

 connected a zone of 

 narrow thick-walled 



FIG. 218 Cordaitean wood from Prince Edward Island elements which form 



