No. 526] SEXUAL REPRODUCTION IN OYMNO SPERMS 603 



Dictyota and Polt/xipliouia, the two generations remain 

 similar throughout the vegetative period, the only dis- 

 tinguishing feature being the number of chromosomes. 



In conclusion, we believe that fertilization is a phe- 

 nomenon of fundamental importance, and that future 

 investigation dealing especially with the differences be- 

 tween the various chromosomes, differences which may 

 be only fortuitous but which may be constant and im- 

 portant, may throw light upon the problems of variation 

 and heredity. That the fusion of gametes always gives 

 rise to a sporophytic generation and necessitates a reduc- 

 tion of chromosomes somewhere in the life history is 

 not so speculative and the claim is readily admitted for 

 plants above the thallophytes. We believe that it holds 

 even for thallophytes. 



In the simplest bryophytes, alternation is already too 

 thoroughly established to throw any light upon the origin 

 of the phenomenon, and the same may well be said of 

 alga? like Dictyota, Cutlcria and Polysiphonia. We be- 

 lieve that even where the first division of the zygote or 

 fertilized egg shows the reduction division, as in Coleo- 

 chcete, there is a true alternation of generations, although 

 the sporophyte generation is very short. The test of a 

 sporophyte is not its longevity. The fertilized egg of 

 a lily is the first cell of the sporophyte, whether it ever 

 divides at all. Consequently, we regard the zygospore 

 of Ulothrix or Spirogyra and the fertilized egg of 

 Vaucheria or (Edogonium as sporophytic structures, 

 even if the first division of the zygote should be meiotic, 

 as seems probable. From such a simple beginning, we 

 believe that the more complex sporophytes with more 

 conspicuous alternation have been developed. The 

 gymnosperms throw no light upon the origin of alterna- 

 tion, but show suggestive stages in the reduction of the 

 gametophytes. They also afford an admirable field for 

 the study of some aspects of fertilization, but we can 

 hardly claim that all the problems of this complex phe- 

 nomenon would be solved with greatest certainty by the 

 study of cycads or pines. 



