202 



THE ANATOMY OF WOODY PLANTS 



part of the wood still sufficiently vouches for the cryptogamic 

 affinities of the lowest living gymnosperms. In Fig. 149 is 

 shown the longitudinal view of the leaf trace in the rachis of the 

 leaf. To the right appears the phloem made up of elongated ele- 

 ments, the sieve tubes. To the left of this region lie certain pitted 

 tracheids belonging to the centrifugal or modern wood. Then inter- 

 vene a few parenchymatous cells, followed by the spiral elements of 

 the centripetal or cryptogamic wood. These pass farther to the left 



FIG. 149. Longitudinal section of a foliar bundle in Cycas revoluta 



into ringed, scalariform, and, finally, pitted tracheids. The struc- 

 ture of the leaf in the Cycadales is clear evidence of their rela- 

 tionship with the ancient fernlike gymnosperms, which in turn were 

 doubtless in filiation with actual ferns in the remote geologic past. 

 The Cycadales have their nearest affinities in the past with a 

 group of gymnosperms, the Cycadofilicales or the Pteridospermeae, 

 which were so strikingly fernlike in their habit that until the very 

 end of the last century they were regarded as ferns. In contrast 

 to the Cycadales the mass of surviving gymnosperms have taken 

 their origin from forms which are known as Cordaitales. These 

 had nothing in their habit which recalls the ferns, but often 



