ISOETACEM 



537 



with membranaceous edges. Just above the base of each per- 

 fectly-developed leaf is a single very large sporangium, sunk 

 more or less completely in a cavity (fovea), which in most 



Fig. 309. — A, Plant of Isoetes Bolanderi, X i ; B, base of a leaf with macrosporan- 

 gium, X4; 'j ligula; u, velum. 



species is covered wholly or in part by a membranaceous indusi- 

 um (velum), and above the fovea is a scale-like outgrowth of 



