PTERIDOPHYTA 



293 



these plants live in water or in wet places and are appropriately called 

 "water ferns." They constitute the most highly developed group of 

 modern pteridophytes. 



1. Marsileaceae 



To the Marsileaceae belong 3 genera. MarsUea, with 65 species, and 

 Pilidaria, with 6 species, are widely distriljuted, while Regnellidinm , with 



Fig. 248. Marsilea vestita. View of plants growing in swanipy ground. 



a single species, is confined to southern Brazil. They live on muddy flats 

 or submerged in water, rooting in the mud. 



Sporophyte. In all three genera the stem is a slender, creeping, 

 branched rhizome that produces erect leaves but no upright shoots. The 

 leaves are arranged alternately in two rows along the upper side of the 

 rhizome, while along the lower side roots are borne at the nodes. Each 

 leaf of Marsilea has a long petiole and four terminal leaflets (Fig. 248). 

 Regnellidium has two leaflets, while in Pilidaria leaflets are wanting, the 

 whole leaf consisting merely of a petiole. As in the Filicales, the leaves 

 exhibit circinate vernation and dichotomous venation. The stem devel- 



