CHAPTER XXVI 

 ALTERNATION OF GENERATIONS 



334. The protoplasmic basis of an alternation of genera- 

 tions.* The history of the alternation of generations in plants 

 has now been traced from the relatively simple beginnings in the 

 thallophytes, as illustrated by the life histories of the red algre 

 (Sec. 246) and sac fungi (Sec. 26,6) through the more clearly 

 denned conditions in the liverworts and mosses, and also through 

 the ferns, horsetails, and club mosses. It is clear that in the 

 latter groups and the pteridophytes the asexual, or sporophyte, 

 generation had become much the more complex of the two, and 

 that the sexual generation, or gametophyte, had begun to degen- 

 erate. This degeneration is carried much further in the seed 

 plants, as will be described in Chapter xxvm, and summarized 

 in Chapter XX ix. 



It is now time to try to determine some of the reasons for 

 the establishment of a sporophyte generation following the game- 

 tophyte one, or the basis in the protoplasm itself of the alternation 

 of sexual and asexual generations. The basis undoubtedly rests 

 on the effects of the sexual process upon the nature of the pro- 

 toplasm in the succeeding generation. The union of gametes is 

 so great a physiological stimulus that the sexually formed cell 

 (generally a fertilized egg) is given the possibilities of a develop- 

 ment different from that of either parent plant or gametophyte ; 

 for the protoplasm of a fertilized egg is not the same as that of 

 either gamete which entered into its formation. It is a mixture 

 of protoplasms and therefore must be different from the proto- 

 plasm of the parent plants, and this difference is the basis for 



* To THE INSTRUCTOR : In a brief course or with immature students this 

 chapter should be omitted. 



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