178 Darling: Sex in dioecious plants 



In view of this belief regarding the determination of sex, at- 

 tention has been directed in the past few years to the study of these 

 cells. Probably no single work has done more to throw light 

 upon this interesting question than the carefully conducted inves- 

 tigations on the dioecious mosses by the Marchals {'06). In ex- 

 periments with Barbiila unguiculata Hedw., Brywn argcnteiim L., 

 and Ceratodoji piirpuretis Brid., they have shown that two kinds of 

 spores exist in every capsule in equal numbers, half of the spores 

 giving rise to male gametophytes and half to female gametophytes. 

 They selected mosses which may regenerate secondary protonemata 

 from fragments of the leaf, stem, or rhizoid of the gametophyte, 

 and in every case the sex character of the parent plant was faith- 

 fully transmitted to the regenerated portions. By subjecting the 

 protonemata to varied conditions of growth, they ^^x^ unable m any 

 case to alter the sex of the individual. These careful experiments 

 seem to confirm their conclusion that the gametophytes in these 

 cases contained the characters of but one sex. 



In a later paper ('07) they report the results obtained in re- 

 searches on the sporophyte. By regenerating parts of the seta 

 or of the walls of the capsule they obtained protonemata which 

 were always bisexual in character, instead of being unisexual as in 

 the case of those produced from the spores or from parts of the 

 gametophyte plant. It is significant that in a great majority of 

 cases these protonemata showed only male characters, a much 

 smaller number showed the sex characters of both male and female, 

 and a very few showed only those of the female. By further ex- 

 periment with secondary and tertiary protonemata obtained from 

 these apparent male and apparent female forms, they found in every 

 case that both sex characters were really present, but that one set 



was 



Since the gametophyte generation has but half as many chromo- 

 somes as the sporophyte generation, they conclude that the uni- 

 sexual character of the spores is due to two series of chromosomes 

 which are separated at the time of the reduction division, or when 

 the spores are formed, so that half the spores contain the series 

 which can develop one sex, and half the series which can produce 

 the other. At the time of fecundation these two series are again 

 united, making the tissue of the sporophyte bisexual in character. 



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