THE FERN ALLIES. 



39 



Louts Acad. Set., IV, 358-390 (1882). A valuable monograph of 

 this most difficult genus of the fern allies. 



UNDERWOOD (L. M.). The distribution of Isoetes. In 

 Botanical Gazette, XI II, 89-94 (1888). 



See also notes in Botanical Gazette, vi, 228. 



E. THE WATER FERNS. 



102. General Characters. This group includes plants 

 of very diverse characters. Some, like Marsilea, root in mud 

 and produce quadrifoliate leaves. Others, like Ptlularia, re- 

 semble the sterile forms of Eleocharis, or other sedges. Others, 

 like Azolla or Salvinia, float on the surface of water, sending 

 numerous roots into the water. Marsilea and Pilularia have a 

 circinate vernation like the ferns. 



103. Fructification. The fruit of Marsilea consists of a 

 hollow-stalked receptacle known as 



the sporocarp, which is oblong or 

 rarely globose, and bears the spo- 

 rangia in sori on the inner walls of 

 its two valves. The spores are of 

 two kinds, as in all rhizocarps. The 

 numerous microspores are con- 

 tained in microsporangia, while the 

 macrospores are solitary in the few 

 macrosporangia. 



1 04. The sporocarp of Pilu- 

 Iraia is globose, containing from 

 two to four cells, which produce 

 microsporangia in the upper portion 

 and macrosporangia below ; the 

 microspores are numerous, while a 

 single macrospore is found in each 

 sporangium. 



1 O5. In Azolla the sporocarps 

 are of two kinds, borne in the axils 

 of the leaves ; the larger are glo- 

 bose, and contain numerous microspores, which are aggregated 

 in masses; the smaller are ovoid, and contain a single macro- 

 spore, 



FIG. 30. Salvinia natans 

 Hoffm.. natural size. (Re- 

 drawn from Thome.) 



