PTERIDOPHYTA 



299 



2. Salviniaceae 



To the Salviniaceae Ijelong 2 genera of widespread occurrence, Salvinia, 

 with 11 species, and Azolla, with 4 species. Both are small plants that 

 float on the surface of quiet water. Their fossil history does not extend 

 beyond the Tertiary. 



Fig. 254. Floating plants of Salvinia rotundifolia (A), natural size, and of Azolla filicti- 

 loides {B), twice natural size. 



Sporophyte. Salvinia has a slender, slightly branched stem bearing 

 two kinds of leaves. The dorsal leaves are in four rows (Fig. 254A). 

 They are 12 to 18 mm. long, entire, oval or oblong, flat, and overlapping. 

 The ventral leaves, which occur in two rows, hang down into the water 



