192 Outline of Genetics 



are separated at the reduction division in the formation of spores. 

 Each spore carries the potentiaHties for one sex only; but of course 

 the sporophyte as a whole before the reduction division must carry 

 the potentialities for both sexes. Marchal, by a peculiar tech- 

 nique of his own, clipped a fragment from a young sporophyte and 

 induced it to reproduce aposporously; that is, the sporophyte 

 fragment produced a gametophyte directly. The fragment must 

 have contained the potentiaHties for both sexes, since it consisted 

 of tissue in which the reduction division had not yet occurred. 

 Presumably, the resulting gametophyte should be bisexual, pro- 

 ducing both antheridia and archegonia, and this was the result 

 actually obtained. It is quite in accord with the sex chromosome 

 theory and a striking confirmation of it. 



If one is to find in plants a sex chromosome mecha- 

 nism comparable to that of animals, he must look to the 

 cases where the diploid sporophyte generation shows a 

 sexual differentiation of individuals, such as in dioecious 

 angiosperms. Santos (19), working with Elodea, has 

 show^n that in the tissues of the male plant there regularly 

 occurs (in addition to certain even pairs of autosomes) 

 one uneven pair of chromosomes, of which the larger 

 member may be designated as X and the smaller member 

 as Y. He has further demonstrated that the reduction 

 division serves to separate the members of this pair, so 

 that half of the pollen grains contain an X chromosome 

 and the other half contain a Y chromosome. PreHmi- 

 nar^^ examination suggests that the tissues of the female 

 plant are regularly of the XX constitution, but this part 

 of the work has not yet been completed. There seems 

 Uttle doubt, however, that here is a sex chromosome 

 mechanism exactly equivalent to those found in the 

 animal kingdom. Two types of pollen grains, X and 

 Y, produced in equal numbers, and mating with (pre- 



