THE FERN ALLIES. 



39 



Louis 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 

 Botaitical Gazette. XIII, 89-94 (1S88). 



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 Pilularia, re- 

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

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

 numerous roots into the watei, 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, wliile the 

 macrospores are solitary in the few 

 macrosporangia. 



1 04. The sporocarp of Pilti- 

 haia 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 05. 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. — Sahiinia nntnns 

 Hoffm., natural size. (Re- 

 drawn from Thome.) 



