THE PTERIDOPHYTES 



291 



foliage leaf. In the sensitive fern (Fig. 241) and ostrich fern 



and some others the sporophyll and foliage branches rise 



separately from the rhizome. This division of labor in fern 



leaves, resulting in development 



of distinct sporophylls and foliage 



leaves, is a great advance. Setting 



apart special structures for special 



pieces of work (division of labor) 



ordinarily increases the quality 



and quantity of work done. 



266. The water ferns. The water 

 ferns are not really members of 

 the true fern class but are closely 

 related thereto. Their water hab- 

 itat is striking. Marsilia (Fig. 

 242) has peculiar leaves, looking 

 like the four-leafed clover, and 

 these float upon water or stand 

 slightly above it. The plant is fairly abundant in greenhouses 

 and park pools. Salvinia (Fig. 243) and Azolla are also widely 

 distributed free-floating water ferns. 



FIG. 243. A water fern (Salvinia) 



The plant has two rows of hairy 

 leaves and one row of water leaves 

 (which look like roots). Natural size 



HORSETAILS OR SCOURING RUSHES (EQUISETINE^E) 



267. General characteristics. This class now consists of one 

 genus (Equisetum) and a few species (variously estimated at 

 from twenty to thirty). The class and closely related classes 

 were once abundantly represented, and as tree-like forms were 

 a prominent part of the earth's flora. Fossil remains of equise- 

 tums and their relatives tell interesting stories of these tree- 

 like forms which lived during the chief coal-forming periods. 

 In studying present-day forms we are observing the remnants 

 of the former abundant plant life of this class. 



Equisetums now live chiefly in regions unfavorable to most 

 plants, around open marshes, in sandy wastes, and along 

 railway embankments. They have hard, rough, siliceous, and 



