PLASTOGENES 



nucleus to carry it on. Development was harmonious because these 

 products were harmonious. Later, when the new and foreign 

 nucleus begins to pass its products into the cytoplasm, the mixture 

 is no longer in harmony and development breaks down. In hybrids, 

 too, such cases may be much more common than those of successful 

 development as in Lens, for they will normally escape detection 

 through their failure to survive. The rare exceptions which are 

 successful enough to be found may give a false picture. 



Plastogenes 



The disharmony that we see between cytoplasm and nucleus in 

 Triton shows that materials of the cytoplasm of the egg must have 

 some permanency, but this need be nothing more than the per- 

 sistence of already existing materials. The experiment is terminated 

 by the death of the hybrid too soon to provide us with the evidence 

 we are seeking of self-propagation in the cytoplasm. Rather than 

 consider the cytoplasm as a whole, we must examine its parts, and 

 the most obvious of these are the plastids which are responsible for 

 the chlorophyll and carotinoid pigments of plants. The plastids are 

 visibly self-propagating bodies in the lower plants and the same 

 property is to be inferred in the higher plants, where they multiply 

 as pro-plastids in a small and colourless stage. The plastids, moreover, 

 contain ribose nucleic acid. Breeding experiments confirm the genetic 

 character first attributed to them by Baur in 1909. 



Primula sinensis, in common with hundreds or thousands of other 

 species of green plants, has a chlorophyll-less or albino variant 

 which is mendelian in inheritance. The homozygote is so pale that 

 it dies; the heterozygote is yellower than the normal, but survives. 

 Thus albinism is here due to something having gone wrong with a 

 nuclear gene which supplies materials necessary for the production 

 or stability of clilorophyll. In the same Primula, however, there is 

 another yellow-leaved variant which passes on its character in a 

 different way. All the offspring from such a yellow mother are 

 themselves yellow, no matter what the character of the pollen 

 parent. A yellow father, on the other hand, does not transmit this 

 character to his offspring. Thus the colour of the offspring depends 

 purely on that of the mother plant and the results of a cross depend 



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