184 Plant Genetics 



the other direction the nucleus would have to manu- 

 facture an additional sex chromosome, which is difficult 

 to imagine. It seems safe to conclude, therefore, that 

 physiological conditions determine sex in some cases and 

 sex chromosomes determine sex in other cases. 



With this background we will consider briefly such 

 work as has been done with plants. In the first place, 

 the sex chromosome has not been established for 

 plants. Sykes, Darling, Strasburger, and others 

 have searched for it diUgently but with negative results. 

 It is true that certain forms show odd chromosomes, 

 such as Ginkgo, Galtonia, Melandrium, Fagopyrum, and 

 Houstonia, but they have never been satisfactorily 

 connected with sex. The most hopeful work has been 

 done by Allen (i) with Sphaerocarpus, which will be 

 mentioned later. It must not be concluded, however, 

 that the chromosome theory of sex determination does 

 not hold for plants. Many cytologists fully expect 

 sex chromosomes to be demonstrated in plants, and at 

 present they point out certain indirect indications of 

 the chromosome situation, such as the even separation 

 of sexes at the reduction division. The theory therefore 

 is firmly believed by many plant cytologists, even 

 though the sex chromosome situation has been estab- 

 Hshed in no plant. Attention may be called to the 

 fact, however, that plants differ from animals in ways 

 that present obstacles to the chromosome theories of 

 sex. Without discussing these obstacles fully atten- 

 tion may be called to two of them. 



In the first place, hermaphroditism is very common 

 in plants, and this is difficult to explain in terms of the 

 sex chromosome theory. In the second place, all the 



