214 THE THEORY OF THE GENE 



autosomes and an XY pair are present in the male. He 

 also found in Vallisneria spirales that in the male there is 

 an unpaired X-chromosome. The formula is Sa+X. 



In Melandrium, Correns has concluded from breeding 

 work that the male is heterogametic. Winge reported that 

 the male formula is 22a+X-f-Y, which confirms Correns' 

 deduction. 



Miss Blackburn also reported an unequal pair of chro- 

 mosomes in the male of Melandrium. She adds one all- 

 important link to the chain of evidence. The female has 

 two equal sex-chromosomes, one of them corresponding 

 to one of the sex-chromosomes of the male (Fig. 123). At 

 maturation they conjugate and reduce. 



From this evidence we may, I think, safely conclude 

 that some at least of the dioecious flowering plants make 

 use of the same kind of mechanism for sex-determination 

 that is present in many animals. 



Sex-Determination in Mosses. 



Several years before these observations on flowering 

 plants had been made, it had been shown by the Marchals 

 that when the spores are formed in dioecious mosses — 

 mosses that have separate male and female gametophytes^ 

 (or sexual prothallia) — two of the spores derived from 



1 In mosses, ferns, and liverworts the haploid or gametophyte generation 

 is spoken of as consisting of two sexes, male and female, and the diploid 

 generation (sporophyte) as non-sexual or neutral. In flowering plants, the 

 plant itself corresponds to the sporophyte of the mosses. It carries, as 

 it were, the gametophyte generation within its pistil and stamens. A paradox 

 arises from the use of the same terms male and female in mosses for one 

 generation, that is, the haploid one, and for the alternative generation in 

 flowering plants, that is, the diploid. The paradox is not so much a question 

 of diploid and haploid (this contrast is encountered even within the same 

 generation in some animals — bee, rotifers, etc.), but in using the same terms 

 for contrasted generations, one sexual, the other non-sexual. With this under- 

 standing, however, no serious difliculty arises by following conventional 

 usage. 



