THE WATER FERNS 



321 



which is briefly described in 

 Sec. 214 and discussed in 

 some detail in Chapter XXVIL 

 The Hydropterales are be- 

 lieved to have been derived 

 from the Filicales, and the 

 development of heterospory is so 

 the most important advance 

 over that group. We can only 

 consider the rather widely 

 distributed type Marsilia. 



313. Marsilia.* Marsilia, 

 the clover leaf fern, or pepper- 

 wort, is easily recognized from 

 the form of the leaf (Fig. 

 281, A). The leaves arise 

 from a creeping stem which 

 in certain species, as M. 

 quadrifolia, grow over the 

 mud in shallow water along 

 the margins of ponds and 

 streams, but often come out 

 of the water upon muddy 

 banks. Other species, as M. 

 vestita, grow almost entirely 

 on muddy banks or in wet 

 meadows. 



The spores of Marsilia are 

 developed in bean-shaped 



FIG. 281. Marsilia 



A, creeping stem of Marsilia quadrifolia, 

 showing a series of leaves in various 

 stages of development: s, spore fruits 

 (sporocarps). B, a spore fruit of 

 M. vestita, which has opened in water 

 and extruded a gelatinous, worm-like 

 structure bearing sori so 



* To THE INSTRUCTOR : If only 

 one heterosporous pteridophyte can 

 be studied in the laboratory, it is 

 much better that the type be Sela- 

 ginella. For this reason the ac- 

 count of Marsilia has been made short. The life-history formula is, of 

 course, the same as that of Selaginella, which is fully treated in Sec. 326. 



