50 



GYMNOSPERMS 



like Cycadeoidca wiclandi to be derived from any of the Cycadofili- 

 cales which have already reached the pitted condition. But if the 

 sequence is due to physiological conditions, especially rate of growth, 

 the kind of markings would have little or no significance as far as 

 relationships are concerned. 



Fig. 44. — Cycadeoidea jenneyana: tangential section of a trunk showing numerous 

 transverse sections of strobili among the foliage leaves; X?. — After Wieland.**" 



We should anticipate much more of the pitted condition than has 

 yet been found; but it must be remembered that Stangcria still re- 

 tains the sclariform tracheid of its very remote fern ancestors, and 

 that Dioon, except in young plants, has sclariform tracheids in the 

 protoxylem. 



