CHAPTEE XXIX 



THE EVOLUTION OF THE SPOROPHYTE AND DEGENERATION 

 OF THE GAMETOPHYTE 



375. The evolution of the sporophyte. Alternation of genera- 

 tions had its beginnings among the thallophytes, and is clearly 

 shown in the life histories of the red algae and the sac fungi, 

 but is not so conspicuous there as in the higher divisions of 

 the plant kingdom. Furthermore, the sporophyte generations of 

 these thallophytes do not seem to be related to the sporophytes 

 of the liverworts and the groups above them, but are probably 

 of independent origin. 



Consequently the line of evolution, with the remarkable 

 development of the sporophyte and degeneration of the game- 

 tophyte, as illustrated by the pteridophytes and spermatophytes, 

 really had its beginning in the lower bryophytes and in the algal 

 ancestry, probably Chloropliycece, from which they were derived. 

 This algal ancestry, however, is not known, for there are no 

 living algse that have the combination of characters which would 

 be expected of the ancestors of the bryophytes, namely, the 

 multicellular sexual organs, together with clearly established 

 sporophyte and gametophyte phases in the life histories. 



The bryophytes were responsible for the first great steps in 

 the evolution of the sporophyte toward the conditions presented 

 in the ferns and seed plants. All of the sporophytes of the 

 liverworts and mosses are to a great extent parasitic upon the 

 gametophytes; that is, they take water from them, and probably 

 certain foods in solution. Two important advances appeared 

 in the bryophytes. 



First. The spore-forming tissue gradually came to occupy a 

 relatively smaller part of the sporophyte (compare the sporophytes 



402 



