CYTOPLASM AND ACTIVATION OF THE GENE 269 



nucleus; but others, noticeably the growth habit, showed a cyto- 

 plasmic influence. This experiment ought to be repeated with 

 the necessary variations to make sure that actually a cytoplasmic 

 effect was involved. It seems probable that it was, in view of 

 the work soon to be reported. 



B. Plastids 



In all these cases, only the general effects of nucleus and 

 cytoplasm were approached by the type of experimentation, 

 without a possibility of isolating the action of individual genes in 

 different protoplasm. 



Now genetic literature is overflowing with analyses of recipro- 

 cal crosses in which the nuclei are alike in regard to individual 

 genes, whereas the protoplasm is different according to the 

 mother. In the overwhelming majority of cases in which the 

 crosses are made between the wild form and its mutations or 

 mutations inter se, no influence of the plasmatic constitution 

 upon the result becomes visible. Reciprocal crosses and their 

 offspring behave alike, of course, with the exception of differences 

 in the X- or Y-chromosomes. We exclude also such differences 

 as find on closer inspection a special explanation. There are, 

 for example, the cases of plastid characters in plants, first studied 

 by Correns, which show inheritance through the mother, as 

 usually no plastids are transmitted by the pollen tube. 



We do not intend to go into the details of the plastid problem 

 here, as we are concerned only with the problem of cytoplasmic 

 heredity. Among the cases studied by Correns (1928) are those 

 in which he came to the conclusion that plastids are prevented 

 from becoming green when situated in a definite cytoplasm, called 

 by him a diseased cytoplasm. But such cases might also find a 

 different explanation. Renner had shown (1924) that a normal 

 behavior of plastids may depend upon definite genetic constitu- 

 tion, as, in Oenothera crosses, plastids within their own cytoplasm 

 will not form chlorophyll in the presence of foreign chromosomes. 

 Following up this work, he came recently (1936) to the conclusion 

 that in all the manifold facts concerning plastid inheritance and 

 behavior, including differences in reciprocal crosses, no cyto- 

 plasmic effect is involved. His opinion is that plastids are 

 independent hereditary units which might even show changes 

 comparable to mutations. The facts of all the different types of 



