338 THE PTERIDOPHYTES 



Isoetes is heterosporous, and the spores are developed in sunken 

 sporangia at the bases of spore leaves (Fig. 291, B, s). The spore 

 leaves are differentiated so that only the outermost develop 

 megaspores and are consequently megasporophylls, while the 



FIG. 291. The quillwort (Isoetes echinospora) 



A, habit sketch. B, base of megasporophyll, showing inner surface : s, sporangium, 

 containing the large megaspores ; I, ligule. C, a group of microspores below, 

 and a large megaspore above, showing comparative size 



innermost are microsporophylls, producing only microspores. 

 Male and female gametophytes are developed slowly in the 

 microspores and megaspores, respectively, and are reduced or 

 degenerate sexual plants (Fig. 292, A, C), almost as simple as 



