xrv 



I SORT AC EM 



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 1 ; B, base of a leaf with macrosporan- 



gium, X4; ^, ligula; v, velum. 



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

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



