278 PHYSIOLOGICAL CENETICS 



under the control of different genes would not suffice to change it. 

 As a matter of fact, experiments arc available to test this problem. 

 The method is to backcross a hybrid that shows maternal 

 influence over and over again with the father, so that the genes 

 are finally mostly paternal, but the cytoplasm is still derived 

 from the original mother. Wettstein performed this with the 

 species crosses of mosses for a number of generations without 

 finding a change in the cytoplasmic influence. Michaelis, how- 

 ever, who performed the same experiment in Epilobium, found 

 that all the characters that showed the cytoplasmic influence 

 continued to do it after eight generations of paternal backcrossing 

 but with a clearly measurable reversion toward the paternal type. 

 This, of course, favors the views of Jollos, which we just 

 represented. 



A similar effect, visible even in F 2 , has been found by Honing 

 (1930) in tobacco crosses, involving hereditary reactions to 

 light. In a more recent paper (Michaelis and Wirtz, 1935), 

 Michaelis minimizes his former results and claims that in spite 

 of a certain reversion the luteum cytoplasm has remained typical 

 after 13 generations with a hirsutum genom. There can be no 

 doubt that the decisive work in this field has still to be done. 



F. Cytoplasm and Sex 



We have mentioned thus far only cases in which ordinary 

 somatic characters were involved. But there are additional 

 facts where the possibility of a cytoplasmic influence in sex 

 determination is under discussion, in both animals and plants. 

 Correns (1928) studied the behavior of gynodioecious plants like 

 Satureia and Cirsium, in which pure females occur besides 

 hermaphrodites, and females pollinated with the pollen of 

 hermaphrodites produce only females. The same result appears 

 if the experiment is performed with different species of Cirsium, 

 one of which is exclusively monoecious; the other, gynodioecious. 

 The result, viz., only female offspring, was not changed if in 

 many succeeding generations the females were backcrossed over 

 again to the hermaphroditic form. From these facts Wettstein 

 derived the interpretation that the female plants contained 

 something in their cytoplasm that inhibits the formation of the 

 male organs. It might be said in favor of such a view that also 



