6 MORPHOLOGY OF GYMNOSPERMS 



in the adult stems of species of Gleichenia (fig. 2), Hymenophyllum, 

 Lygodium, and the paleozoic Botryopteris (36). 



Which of the two kinds of siphonostele has been derived directly 

 from the protostele is a question under discussion. One view (31) 

 is that the amphiphloic condition was the first to appear, and that by 

 the gradual reduction of the internal phloem the ectophloic stage 



Fig. 3. — Adiantitm pedatiim: transverse section of stem, showing the amphi- 

 phloic siphonostele; X25. — After Coulter (88). 



was reached. The other view (36) is that pith appeared first in the 

 solid stele (protostele), without any internal phloem, which would 

 make the ectophloic siphonostele more primitive than the amphiphloic. 

 The investigators concerned with these views have arranged such 

 convincing series that it seems reasonable to suppose that both kinds 

 of siphonostele may have arisen directly from the protostele. In any 

 event, the sporeling of such a fern as Adiantum begins with a pro- 



