6o2 A TEXTBOOK OF THEORETICAL BOTANY 



some other species may have twice that number. As in the megaspor- 

 angium, there is no dehiscence of the sporangial wall. 



|_^ Ligule 



Glossopodium ^ 



Trabecula 



Velum 



. Sporangium 



Fig. 6i2. — Isoetes lacustns. Longi- 

 tudinal section of microsporo- 

 phyll showing the structure 

 of the ligule and microspo- 

 rangium with trabeculae. 



The Male Gametophyte 



The microspores are bean-shaped with smooth walls. They do not begin 

 germination until after they are set free, but they do so most promptly, though 

 rather irregularly, in winter, when the male prothallus ripens in about two 

 weeks from shedding. This is probably the natural time at which the 

 sporangium decays. In the first place a small prothallial cell is cut off at 

 one end of the spore, the rest of the spore forming the antheridium (Fig. 613). 

 Obliquely intersecting walls divide it into four peripheral cells and four 

 central ones, each of the latter forming a single-coiled antherozoid, which, 

 like those of the Ferns, and unlike those of Lycopodiales, has a tuft of fifteen 

 flagella (Fig. 614). The peripheral cell walls dissolve, leaving the antherozoids 

 free inside the spore. They are liberated by the eventual dehiscence of the 

 spore wall along the triradiate ridge. 



