126 



THE MARATTIALES 



A midrib usually begins to form at an early period and in the older prothallia 

 may become very conspicuous, sometimes reaching a thickness of eight to ten cells. 

 The margin, as we have seen, is always more or less irregular in outline and often 

 develops large leaf-like lobes vv^hich are particularly conspicuous in D. jamatcensts 

 (fig. 91, C), but are noticeable also in the other species. The large prothallia, which 

 are sometimes nearly 3 cm. in length, are often branched (fig. 92, D); the branching 



Fig. 94. 



A, archegonium; B, antheridium of Marattia douglasii. n, neck canal cell ; 

 r, ventral canal cell; o, egg; m, mande cells. 



is sometimes unmistakably a true dichotomy, but again it seems to be adventi- 

 tious. In one large prothallium of Z). jamaicensis of very irregular form, four groups 

 of archegonia were present, widely separated from each other. It could not be cer- 

 tainly determined whether these arose from a repeated forking of the original apex 

 or whether one or more of them was of adventitious origin (fig. 91, C). 



Fi... 95. 



\nthfr\A\\imoi Anglopleris. X275. ra, mantle celli; t/, operculum. 



A, B. Longitudinal sections. C. Transverse section. D, E. Surface view. 



The apex of the prothallium in the younger stages is usually indented by a 

 sinus, but in the older ones the heart shape is almost completely lost and the apex 

 may even project as a rounded protuberance, bearing the archegonia upon its lower 

 surface, or it may be fan-shaped, with little or no indentation at the growing point. 



The prothallium is the usual dark-green color found in the other Marattiaceae, 

 but as we have seen is rather more delicate in texture. Of the three species 

 examined, Dantsa elliptica resembles more nearly the prothallia of the other Marat- 

 tiaceae in the thickness of the central portion, which may show from eight to ten 



