ALTERNATION OF STRUCTURAL TYPES. 13 



include synapsis and the subsequent heterot3'pe and homotj'pe nuclear 

 divisions." 



ALTERNATION OF STRUCTURAL, TYPES. 



"Alternation of generations" is an expression borrowed from zool- 

 ogy; its application to the archegoniate plants has introduced endless 

 complexities, and can be justified, after all, only by false analogies. 

 Alternation of generations was discovered b\^ Chamisso in a species of 

 Salpa, a marine animal belonging to the Tunicates; but here, as well 

 as in the ti-aditional zoological example of the Aphides, or plant lice, 

 the phenomena have entirely different evolutionary significance from 

 the so-called antithe'tic alternation of generations in the archeo-oniate. 

 Generations or individual life cycles of Salpa and of plant lice, which 

 were original!}' alike, have become different, so that now partheno- 

 genetic generations alternate with sexual generations. To make the 

 archegoniate plants a jmrallel instance, it would be necessary to assume 

 that what is now called the sporophyte was originally another thallus, 

 or something that corresponded to one, which later on became modified 

 into the sporophytic "generation." To state the case in this way may 

 seem quite su]3erfluous, since nobody has made such a suggestion. 

 Stras])urger and others have repeatedly declared that the so-called 

 asexual generation had been intercalated — that is, added anew — and 

 not substituted for something else. This, however, makes it only the 

 more obvious that the sporophyte is a generation onh' in a very loose 

 and inaccurate sense, and not because it corresponds to or takes the 

 place of any other generation. The simple fact is that, instead of form- 

 ing merely one oospore as the result of fertilization, the archegoniates 

 have come to form a whole sporophyte or double-celled structure by 

 the multiplication and progressive sterilization of potentially sporoge- 

 nous tissue, as Bower has shown. '^ 



Bower's generalization is, however, only a half truth, since the sterili- 

 zation, or, better, the arrest of spore formation of some of the cells, is 

 conditioned on the possibility of continued subdivision and growth of 

 the fertilized egg, and this can occur only when there is a definite 



«To recognize, liowever, as Farmer and Moore do, these two cell generations as a 

 distinct "maiotic pliase" in the life history of Metaphyta and Metazoa does not 

 seem warranted, since chromosome reduction is apparently a mechanical necessity 

 resulting from sexual conjugation and is consequently brought about in a practically 

 identical manner in all symbasic organisms, from the lowest to the highest. Maiosis 

 is rather a connecting link at the node in the network of descent than a distinct 

 phase subject to expansion or contraction as organisms mount in the scale of evolu- 

 tionary progress. On the other hand it is clear that the two peculiar cell generations 

 occurring during maiosis can not proi)erly be clas.sed with the double-celled pha^^e 

 that usually precedes or with the simple-celled phase that usually follows, but con- 

 stitute a transition stage marking the end of one generation and the beginning of 

 another. 



''Bower, F. O. A Theory of the Strobilus in Archegoniate Plants. Annals of 

 Botany, 8:343-365. 1894. 



