CHAPTER XI 



LEPTOSPORANGIAT^ HETEROSPORE^ (HYDROPTERIDES)^ 



The two very distinct families of heterosporous Leptospo- 

 rangiatse 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 Marsiliaceae are 

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

 although possessing many points in common. They are. all 

 aquatic or amphibious plants, and the gametophyte, especially 

 in the Marsiliaceae, is extremely reduced. 



Salviniace^ 



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

 small floating aquatics which differ very much in the habit of 

 the sporophyte from any of the other Filicinese, but in the de^ 

 velopment of the sporangia and the early growth and form of 

 the leaves show affinities with the lower homosporous Lepto- 

 sporangiatse, from some of which they are probably derived. 



The fully-developed sporophyte is dorsiventral, and the 

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

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

 overlap, so that they quite conceal the stem. Roots are devel- 

 oped in Azolla, but are quite wanting in Salvinia, where they 

 are replaced physiologically by the dissected ventral leaves 

 (Fig. 233). The sporophyte branches extensively, and these 

 lateral shoots readily separate, and in this way the plants multi- 

 ply with extraordinary rapidity. The sporangia are enclosed 

 in a globular or oval "sporocarp," which is really an indusium, 



' Also known as Rhizocarpeae. 



396 



