218 Cytoplasm as Seat of Genetic Properties 



a little trick of making the haplont diploid. The pollen sterility in 

 tliis combination remained constant. Wettstein considers this to be 

 real proof of an independent plasmon, though, I think, an interpreta- 

 tion of different genie interaction with different cytoplasmic sub- 

 strate is not excluded. 



In the vast collection of data found in the work of the schools 

 of Renner, Lehmann, Schwemmle, and Michaelis, innumerable vari- 

 ations in tlie quantitative behavior of the traits affected by the 

 cytoplasm were found, but not a single convincing case of genuine 

 plasmonic, maternal control of a definite hereditary trait. It is very 

 doubtful whether such a type of heredity exists, at least outside the 

 sphere of sex determination. 



All this is very important for the problem of so-called cytoplasmic 

 genes, which had become a kind of genetic fashion but are in eclipse 

 again at present. If the mitochondria play the role we discussed 

 previously, they cannot be called genes, since their action can be 

 only a generalized one, with possible differences in regard to their 

 quantity. There is no reason to suppose that different types of mito- 

 chondria exist which can be sorted out, each kind controlling a 

 definite character. In the same way, the assumption of a plasmon, 

 which could be conceived of as an assembly of cytoplasmic genes, 

 would require different kinds of cytoplasm, separable and endowed 

 with definite hereditary actions. No such case has been analyzed 

 thus far, and it is extremely improbable that one wOl turn up. 

 Of course, differentiated cytoplasm which controls the fate of nuclei 

 within it (or of Lwoff's kinetics) is completely different (see I 1). 

 It is true that Darlington (1944) once tried to interpret some of the 

 features of Michaelis' Epilobium crosses as indicative of cytoplasmic 

 segregation and Michaehs ( 1954 ) insists upon it. The zoologist will be 

 extremely skeptical if he thinks of cytoplasmic differentiation in de- 

 velopment, which is certainly different from genetic segregation. 

 Thus I consider it a fact that so far genuine cytoplasmic heredity (i.e., 

 not of the substrate-for-genic-action type), or the existence of what 

 could be called cytoplasmic genes, has not been proved, with the 

 probable exception of stuffs related to special features of sex determi- 

 nation in Streptocarpus. This conclusion applies thus far only to the 

 facts which have just been analyzed; none of them deal with visible or 

 invisible but analyzable cytoplasmic substances of a particulate type, 

 though we came near this subject when discussing mitochondria. 



