272 PHYSIOLOGICAL GENETICS 



cioss generation which behaved according to the expectation 

 derived from the given explanation. In the generic cross, no 

 such direct way of testing was possible on account of the sterility. 

 But it was to he expected that among those spores which are 

 normally formed, because the respective gene recombinations 

 are viable, no difference of viability would exist between gene 

 recombinations of predominantly paternal or maternal origin. 

 As a matter of fact, more maternal recombinations were found 

 in the reciprocal crosses, and this is interpreted as meaning 

 that the genes work more successfully with the cytoplasm of 

 their own species. In this cross, it was also possible to backcross 

 over again in succeeding generations with the paternal species, 

 but the result was always maternal offspring, which shows that 

 the supposed plasmatic incompatibility with the foreign chromo- 

 somes has not been changed. Unfortunately, also, in this case 

 the decisive experiment with extracted homozygotes in recipro- 

 cally different cytoplasm could not be performed, except for 

 one generation of a backcross on one side only (see page 275). 



The plasmatic influence that was found in these more com- 

 plicated crosses was tested in many different ways by Wettstein 

 and his students (Doerries-Rueger, 1929; Becker, 1931; Melchers, 

 1935) and was found to apply to many individual characters like 

 osmotic pressure and division rate. All these cases may, however, 

 be interpreted as a general action of the cytoplasm upon gene- 

 controlled reactions. No influence has been found that could 

 properly be called a cytoplasmic determination of a definite 

 trait. 



One more case of a similar type in plants might be mentioned, 

 because it shows again certain of the general features that we 

 have met thus far. Sirks (1931a, b, c) found in his crosses with 

 beans no reciprocal difference of crosses within the two sections 

 of vicia faba called major and minor. But in crosses between 

 these sections, differences came to light that have to be regarded 

 as of plasmatic nature. If a certain linkage group of genes is 

 represented in a cross, the homozygous forms appear only in F 2 

 if the genes in question are situated in the protoplasm of the form 

 from which they entered the cross. Otherwise, they are not 

 viable. In other words, the protoplasm of the major group is 

 lethal for a certain chromosome derived in homozygous form 

 from a minor plant. Or, stated another way, the chromosome 



