THE ADVANCE IN PLANT EVOLUTION 307 



algse and soon became established as the vegetative period in 

 the plant's life history; (2) as a result of this, the motile 

 stages (zoospores) became set apart as reproductive phases in 

 the life histories, such reproductive motile stages, with other 

 reproductive cells, being called spores ; (3) certain of the repro- 

 ductive cells became sexual in character, and these gametes at 

 first similar (isogamy) were later differentiated into eggs and 

 sperms (heterogamy) ; (4) alternation of generations developed 

 hi the red algse and sac fungi, but probably independently of 

 the same phenomenon in the bryophytes. 



2. The fungi. The fungi as special and peculiar offshoots 

 from the algse have of course contributed nothing to' the main 

 evolutionary line running up to the higher plants. 



3. The liverworts and mosses. The chief advances of the 

 liverworts and mosses over the algse were three in number: 



(1) many-celled sexual organs (antheridia and archegonia) took 

 the place of the one-celled reproductive organs of the algse; 



(2) an alternation of generations (gametophyte with sporo- 

 phyte) became well established, together with the origin of a 

 new type of asexual spore developed in groups of four (tetrads) 

 by the sporophyte ; (3) there was a general advance in the cell 

 structure of the plant bodies because of adaptations to the more 

 complex conditions of the land habit. 



The sporophyte of the bryophytes is always attached to the 

 gametophyte, and except in Anthoceros and some mosses it is 

 not as complex as the gametophyte. In the pteridophytes, how- 

 ever, the conditions are reversed. The sporophyte is the large, 

 conspicuous phase in the life history, and as it develops it becomes 

 entirely independent of the gametophyte, while the latter appears 

 relatively insignificant, although it holds, of course, a necessary 

 place in the life history. The appearance in the pteridophytes 

 of the sporophyte as an independent plant was the most impor- 

 tant advance in plant evolution at this time, for the vegetative 

 activities gradually became shifted, at first chiefly and finally 

 wholly, from the sexual to the asexual generation. 



