296 



PLANT MORPHOLOGY 



and develops a very thick cell wall, as in the other heterosporous pterido- 

 phytes. In both kinds of sporangia the tapetum breaks down to form a 

 multinucleate plasmodium that surrounds and nourishes the young 



spores. 



The sporocarp of Marsilea is remarkable on account of its longevity, 

 some specimens having been known to have retained their viability for 

 50 years. If placed in water after the hard outer covering has been 



Fig. 251. Germination of the sporocarp of Marsilea vestita, twice natural size. 

 of mucilaginous ring to which the sori are attached. 



Extrusion 



cracked, germination is unusually prompt. Within an hour a mucilag- 

 inous ring appears to which the sori are attached, each sorus being 

 enclosed by its own indusium (Fig. 251). The spores germinate at once 

 and the gametophytes develop with startling rapidity. 



Gametophytes. The male gametophyte of Marsilea reaches maturity 

 within 10 to 20 hours after the microspore germinates. It does not 

 emerge from the spore, but develops inside, as in SelagineUa and Isoetes. 

 After a prothallial cell is cut off, the rest of the spore divides in half, each 

 half becoming an antheridium (Fig. 252.4-C). Additional divisions 

 result in the formation of two primary spermatogenous cells surrounded 

 by a sterile jacket (Fig. 252D-F). Each of the primary spermatogenous 

 cells gives rise to a group of 16 sperms (Fig. 252G-I). The sperms of 

 Marsilea are corkscrew-like and multiciliate (Fig. 252 J). 



The female gametophyte of Marsilea is peculiar in that no internal 



