THE PTERIDOPHYTA : FILICALES, THE FERNS 553 



vacuole becomes divided by partitions into a number of alveoli. Each group 

 of spores thus enmeshed in the alveoh is called a massula (Fig. 558). From 





Fig. 558. — Azolla filicidoides. Massula 

 after liberation showing hooked 

 glochidia and alveoli containing micro- 

 spores. 



the wall of the enclosing vacuole there grow out a number of radial processes, 

 like hairs, which end in barbed hooks. These extraordinary structures, 

 formed from a vacuole membrane, are called glochidia, and they function 

 by attaching the liberated massulae to the megaspore. 



Development of the Megasporangium 



In the megasporangium the lower part of the sporangial wall first of all 

 disintegrates, leaving the single large megaspore at the bottom of the sorus 

 covered only by the indusium. The tapetum then forms a plasmodium, as 

 in the microsporangium, which divides into four massulae. One of these 

 envelops the megaspore, forming a sculptured epispore, marked by cavities 

 and projecting processes. The other three massulae are grouped above the 

 megaspore. They become highly vacuolated and harden into spongy bodies 

 called collectively the swimming apparatus (Fig. 559). 



Inside the indusium, above the megaspore, there are present numerous 

 spores of A?iabaena. These are shed along with the megaspore and germinate 

 with it so that the association of the two plants is kept up from one generation 

 to the next. 



The indusium partly disintegrates, liberating the megaspore and its 

 appendages, but the upper portion and the remnants of the sporangial wall 

 remain attached to it, forming a kind of umbrella. The microspore massulae 

 are similarly liberated and the glochidia hook themselves to the sculpturing 

 of the epispore round the megaspore, thus ensuring the proximity of the 

 prothalli. 



