48 



GYMNOSPERMS 



cal attention, for the Cycadofilicales had so generally reached the 

 pitted condition. The question may well be raised whether a plant 

 whose tracheids have reached the pitted condition could give rise to 



Fio. 41. — Cycadeoidea wielandii: transverse section of secondary wood; X150. 

 After WiELAND.**" 



Fig. 42. — Cycadeoidea wielandii: transverse section of phloem showing thin-wallcd 

 cells and thick-walled tracheids; X 150.— .\ftcr Wieland.'*' 



a line with sclariform tracheids. Atavism is a familiar phenomenon; 

 but, as we understand it, its influence is potent only for a short time, 

 not persisting from one geological age to another. If the sequence, 

 spiral, sclariform, pitted, is fundamental, we should not expect a form 



