170 THE SALVlNIACEyE. 



have arisen through some such ancestor as the floating 

 fern, though as a matter of present relationships the 

 species of this group appear to have closer affinities for 

 the filmy ferns (Hymenophyllacea;), especially as regards 

 the formation of their sporocarps. In any event they are 

 probably more closely allied to the ferns than any of the 

 other fern allies except the Marsiliacea;, and some are 

 so much like ferns in their structure as to have received 

 the common name of water-fern. As a class they are, 

 with the Marsiliaceje, sometimes spoken of as the 

 Hydropterids (which may be translated as water-ferns), 

 or as Rhizocarps, which refers to the way the sporocarps 

 are borne in these families. 



The Salviniaces contain but two small genera, Sal- 

 vinia and Azolla, and these differ markedly in structure. 

 All of them, owing to their position on the water, are 

 dorsi-ventral, the stem growing horizontally. In Azolla, 

 roots are sent down into the water, and at first glance 

 this seems to be the case in Salvinia also, but a closer 

 inspection shows that what appear to be roots in this 

 genus are really dissected leaves and that the species are 

 rootless. Above the water the vegetative leaves spread 

 out, those of Salvinia oblong and entire, while those of 

 Azolla are cut into more fern-like shapes. In most 

 species numerous lateral branches are given off by the 

 main stem, and these readily separate from the plant 

 when the water is agitated, and form new plants. A 

 rough day on the water, therefore, greatly assists in 

 multiplying their numbers. 



The spores are of two kinds, as in Selaginella and 

 Isoetes, and are borne in structures called sporocarps, or 

 conceptacles which appear to be special forms of an 

 jndusium. They are much like what would result if the 



