i go Plant Genetics 



PTERIDOPHYTES. In this group we are confronted 

 by a new complexity. In the preceding groups it was 

 necessary to consider separately monoecious and dioe- 

 cious forms, assuming that in the latter the sexes are 

 separated at the reduction division. In the higher 

 Pteridophytes, however, heterospory appears, and 

 establishes once for all the dioecious condition in the 

 gametophyte generation. In heterosporous forms there- 

 fore the gametophytes are always dioecious, but it can- 

 not be said that the sexes are separated at the reduction 

 division. Obviously no such claim can be made, for it 

 would have to assume that each tetrad consists of two 

 megaspores and two microspores, a thing which prob- 

 ably never occurs, unless possibly in Equisetum, which 

 should be investigated. It is safe to conclude therefore 

 that sex in heterosporous forms is differentiated before 

 the reduction division. A megasporangium produces 

 only megaspores, although in doing so the reduction 

 division occurs. In Selaginella, which is perhaps the 

 best example, one can distinguish a megasporangium 

 from a microsporangium as soon as the mother-cells 

 are formed. Clearly the sexes are separated before 

 the reduction division. In attempting to explain this 

 situation cytologists claim that the sexes are separated 

 at one of the vegetative divisions of the sporophyte 

 previous to the formation of mother-cells. This is 

 merely an assumption and an extremely unsatisfactory 

 one, for only in connection with the reduction division 

 is there a proper mechanism for the separation of sexes 

 according to the sex chromosome theory. Furthermore, 

 at present there is just as good reason to believe that sex 

 in Selaginella is determined by physiological conditions. 



