156 Derby — Crown Structure of Pscronius Braziliensis. 



tural features of stem and crown, is that they descend from the 

 first part of the outspreading portions of both the P and F 

 organs. 



An argument in favor of this hypothesis can be drawn from 

 the history of the F organs as traced above. Those in middle 

 life have their front nearly covered by a sclerotized layer 

 which cleaves away when they commence to bulge over the 

 rim of the stem rind, while at the same time the section of the 

 vascular strand that lay behind the bulging front shows a sud- 

 den interruption. It seems reasonable to assume that this 

 interruption may be due to the subdivision of the band-like 

 strand into numerous filaments to supply radicular appendages 

 to the lower face of the outspreading stalk, while the lateral 

 portions of the original U-shaped strand continue for a farther 

 distance outward before subdividing to supply the more essen- 

 tial appendages of the stalk. When the organ fell away the 

 breaking point would be at the outer margin of the root-bear- 

 ing portion, but the part of the stalk above the lower face 

 would be exposed to the air and would decay, while the verti- 

 cal portion behind it would be protected by the roots and thus 

 preserved as persistent stubs. If this was so, the stem would 

 grow upward free from roots until the succeeding organ of 

 the same row sent down a fresh lot to tie on to the retarded 

 upper rim of the root sheath. In the case of the P organs the 

 central part of the vascular strand may be presumed to subdivide 

 in a similar manner at the point of outbending, while the lat- 

 eral portions go on to supply the pinnules that are only 

 developed farther away. The well-known occurrence of modi- 

 fied pinnules (?), known as Aphlebia, on the stalks of many 

 fossil ferns (?) may perhaps furnish an argument in favor of 

 this hypothesis. 



Rio de Janeiro, March 15th, 1914. 



