PTERIDOPHYTA 



295 



appear in the young sporocarp of Marsilea and from the layer of cells 

 lining them the sporangia arise, each sorus coming from a single marginal 

 cell (Fig. 250A). The sporangia appear in hasipetal succession on an 

 elongating receptacle, the sorus thus being gradate, as in certain families 



Fig. 250. Sporangia of Marsilea quadrifolia. A, horizontal section through a young 

 sporocarp, showing early development of two rows of sori, X25; B, longitudinal section 

 through a young sorus and investing indusium, showing a developing megasporangium with 

 sporogenous tissue and tapetum, and two younger microsporangia below, each with a pri- 

 mary sporogenous cell, X400; C, megasporangium with young megaspores and tapetal 

 Plasmodium, X300; D, megasporangium with enlarging functional megaspore, abortive 

 megaspores, and tapetal plasmodium, X300; r, receptacle; h, vascular bundles; i, indusium; 

 s, stalk of sporocarp. 



of the Filicales (see page 286). The tapetum, cut off from the sporog- 

 enous tissue, becomes two-layered (Fig. 2505). In both kinds of sporan- 

 gia 32 or 64 young spores are formed. In the microsporangia all of these 

 mature, but in the megasporangium only one spore matures, the others 

 degenerating (Fig. 250C, D). The functional megaspore greatly enlarges 



