CHAPTER XV 



THE NATURE OF THE ALTERNATION OF GENERATIONS 



The origin and significance of the phenomenon of the alterna- 

 tion of generations, so characteristic of the Archegoniates, and 

 its bearing upon the origin of the leafy sporophyte of the higher 

 plants, have been the subject of much discussion. 



Among the lower plants the phenomenon is not uncommon, 

 but it is in none of these so prominent as it is among the Arche- 

 goniates. If the views of Oltmanns (2) are accepted, the 

 cystocarp of the Rhodophyce'ae represents a neutral generation, 

 comparable in a way to the sporophyte of the Archegoniates, 

 and like the sporophyte of the Muscineae is parasitic upon the 

 gametophyte. The fruiting body resulting from the fertilisa- 

 tion of a carpogonium or archicarp in many Ascomycetes also is 

 very similar to the cystocarp of the Rhodophyceae, and might 

 perhaps with equal propriety be denominated the sporophyte. 



The method of development of the sporophyte in these 

 forms, however, is very different indeed from that of the Arche- 

 goniates, and does not suggest even a remote homology. 



Among the Chlorophycese, the alternation of generations is 

 not conspicuous, but it is nevertheless in this group and not 

 among the Rhodophyceae that we are to seek the progenitors of 

 the Archegoniates. 



The presence of sexual and non-sexual plants among the 

 Green Algae is in no way comparable to the alternation of game- 

 tophyte and sporophyte in the Archegoniates. The same indi- 

 vidual in Oedogonium or Vaucheria may produce either zoo- 

 spores or gametes, and the production of sexual or non-sexual 

 cells is largely due to external conditions. ( See Klebs ( i ) ) . 

 The product of the fusion of the gametes in these plants is a 

 resting spore, which on germination, either directly or by the 



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