172 THE SALVINIACE/E. 



to science, most of them in tropical waters. The name 

 Azolla is derived from two Greek words meaning " to 

 dry " and " to destroy," and is most appropriately 

 applied to these delicate little plants whose life is so 

 closely dependent upon moisture. There are less than 

 half a dozen species in the world. Like the salvinias 

 they delight in the warm waters of tropical regions. The 

 species in both genera are very small, the largest having 

 leaves not more than two inches long, while in a majority 

 they do not reach a length of half an inch. All are 

 supposed to be short-lived, possibly annual plants, but a 

 few seem to live to a greater age. A dozen or more 

 species of Salviniah.a.ve been found in a fossil state, but 

 usually in deposits of no very great age. 



