THE PTERIDOPHYTA : FILICALES, THE FERNS 



551 



development begins with the formation of a megasporangium at the tip of 

 the placenta and microsporangia round its base (Fig. 555). In a megasorus 

 the megasporangium contains thirty-two potential megaspores, but all except 

 one abort, as well as the microsporangia. In a microsorus all the megaspores 



INDUSIUM 



ANABAENA CELLS 



MEGASPORANGIUM 



ABORTIVE 

 MICROSPORANGIUM 



Fig. 555. — AzoUa filiculoides. Longitudinal 

 section of a developing megasorus showing 

 abortive microsporangia. {After Goebel.) 



abort and the microsporangia continue development. The result is that a 

 megasorus contains a single megasporangium with a single megaspore, and 

 a microsorus contains numerous long-stalked microsporangia, containing 

 sixty-four microspores * (Figs. 556 and 557). 



Development of the Microsporangium 



The developments inside the sporangia are highly peculiar. Let us take 

 the microsporangium first. Sixty-four spores are formed, surrounded by a 

 nutritive wall of cells, the tapetum, which forms the inner layers of the 



* In some species the sporangium is completely encircled by an oblique ring of cells 

 which probably represents a vestigial annulus. 



