382 GENERAL MORPHOLOGY OF PLANTS 



(Class Filicinez) the progression has gone on as illustrated by 

 the horsetails and the club mosses until a condition is reached 

 which is very much like that of the seed plants. The principal 

 things which have been developed among the fern plants and 

 which mark the progression above the liverworts may be enume- 

 rated as follows: 



First. The sporophyte has become an independent plant by 

 the development of roots, of special organs for assimilation 

 (leaves), and of a well- developed vascular system for the trans- 

 port of water and food materials, as well as the differentiation of 

 other tissue systems. 



Second. Alternation of generations has reached its highest 

 expression in that both generations, the gametophyte and the 

 sporophyte, can live as independent plants. 



Third. Dimorphism of the leaves of the sporophyte which 

 results in a division of labor among the leaves into those for the 

 function of photosynthesis and those for spore bearing. This 

 was begun in a few of the ferns, becomes the rule in the club 

 mosses, but is not present in Isoetes. 



Fourth. Heterospory, the development of two kinds of spores 

 large and small, has originated in the higher fern plants. This 

 predetermines the sex of the gametophytes and insures cross 

 fertilization among the gametophytes, which is a distinct advan- 

 tage, and is one of the characteristics of the seed plants. This 

 dimorphism (or heterothallic condition) of the gametophytes is 

 foreshadowed in the true ferns where varying amounts of nutri- 

 ment may determine the sex of the prothallia; it is almost 

 wholly determined in Equisetum. In Selaginella and Isoetes it 

 is predetermined in the spore. 



Fifth. The sporophyte is the most prominent part in the life 

 cycle of fern plants and is better adapted to existence on the land. 

 It has a decided advantage over the gametophyte generation which 

 is especially adapted to wet or moist situations, and which requires 

 water as a medium for conveying the sperms to the egg. The 

 earliest green plants, the algae, are almost exclusively of an 

 aquatic habitat. The sporophyte of the fern plants being per- 



