CHAPTER XII 



LEPTOSPORANGIAT^ HETEROSPORE^ (HYDROPTERIDES)^ 



The two very distinct families of heterosporous Leptospor- 

 angiatae have obviously but little to do with each other, but, 

 both of them being evidently related to the homosporous forms, 

 they may be placed together for convenience. Each of the two 

 families contains two genera, which in the Marsiliace^e are 

 closely allied, but in the Salviniaceae not so evidently so, 

 although possessing many points in common. They are all 

 aquatic or amphibious plants, and the gametophyte, especially 

 in the Marsiliacese, is extremely reduced. 



Salviniace(E 



The two genera, Salvinia and Azolia, contain a number of 

 small floating aquatics which differ very much in the habit of 

 the sporophyte from any of the other Filicineae, but in the 

 development of the sporangia and the early growth and form 

 of the leaves show affinities with the lower homosporous 

 Leptosporangiatae, from some of which they are probably 

 derived. 



The fully -developed sporophyte is dorsiventral, and the 

 leaves are arranged in two dorsal rows in Azolia, four dorsal 

 and two ventral in Salvinia. The dorsal leaves are broad and 

 overlap, so that they quite conceal the stems. Roots are 

 developed in Azolia, but are quite wanting in Salvinia, where 

 they are replaced physiologically by the dissected ventral 

 leaves (Fig. 196). The sporophyte branches extensively, and 

 these lateral shoots readily separate, and in this way the plants 



^ Also known as Rhizocarpese. 



