PTERIDOPHYTA 



301 



spores are ripe, forming a plasmodial matrix around them (Figs. 2555 and 

 256C, D). Eventually this hardens. 



In the microsporangium of Azolla four to eight masses, called massulae, 

 are organized from the tapetal plasmodium and within these the micro- 

 spores are embedded. The massulae of some species of Azolla produce 

 hair-like appendages (glochidia) with sagittate tips (Fig. 257A). They 



B C 



Fig. 255. Sporangia of Azolla caroliniana. A, young sporocarp, showing a young mega- 

 sporangium and the developing indusium; B, megasporocarp, showing the terminal mega- 

 sporangium with one functional and three abortive megaspores, and undeveloped micro- 

 sporangia below; C, microsporocarp with developing microsporangia and an abortive 

 megasporangium. {After Pfeiffer.) 



escape from the microsporangia and are carried to the megaspores, to 

 which they become fastened by means of the glochidia. The microspores 

 germinate within the massulae. In Salvinia they germinate while still 

 within the microsporangia. The megaspores remain inside the megaspo- 

 rangia, which break away and, in Salvinia, float on the surface of the 

 water. 



Gametophytes. The male gametophyte of the Salviniaceae is peculiar 

 in that the microspore produces a papillate outgrowth that forms one or 

 two external antheridia, the internal portion functioning as a large nutri- 

 tive cell (Fig. 2575). In both genera a small prothallial cell is cut off 

 from the nutritive cell. In Salvinia, where there are two antheridia, each 

 is enclosed by a sterile jacket and each produces four sperms. In Azolla 



