PLASMAGENES 



correct nucleus. The reciprocal cross, green by variegated, gives 

 exactly the same result except that there are, of course, no white 

 eggs from the green mother and no white seedlings in the F^. In 

 this cross the plastids which begin to mutate in the Fg have no 

 ancestry of mutability. The mutability of the plastids is the same, 

 no matter what their ancestry may have been. It is determined by 

 the nucleus, although their character before and after mutation is 

 determined by themselves within the range of nuclear variation 

 studied in this experiment. 



This kind of situation (which is known also in maize and rice) 

 puts control and independence in a new light. They are not so simple 

 as they might seem to be. They exist in three spheres of action. The 

 first is reproduction, and here each organ of the cell is controlled 

 by the whole. The second is activity or production, and here again 

 control is by the whole, but in this control the cytoplasm is the 

 immediate, and the nuclear genes together with extra-nuclear genes 

 the ultimate authority. The third is mutation, which is similar in 

 principle to the second, although the detailed processes are entirely 

 different and the predominance of the nucleus is even more obvious. 

 We may indeed speak of certain genes inside the nucleus as being 

 mutafacient with respect to particular other genes, either inside or 

 outside the nucleus. 



Plasmagenes 



Not all extra-nuclear determinants are conveniently attached to 

 markers so easy to pick out as plastids, or so clear in what they do. 

 Their existence, however, is vouched for by the non-mendelian 

 inheritance, of which reciprocal differences are the simplest evidence, 

 appearing in a great number of crosses in flowering plants, ferns, 

 mosses, and protozoa. 



The moss Fiinaria provides an example of a reciprocal difference 

 in crossing. Crosses made by Wettstein between F. mediterranea and 

 F. hygrometrica differ in the shape of the diploid structure, the sporo- 

 carp, according to the direction of the cross. A diploid gametophyte 

 can be produced from the sporocarp by regeneration following 

 injury. Meiosis and segregation are thus avoided. Like the sporocarps 

 whose unchanged nuclei they bear, these gametophytes differ in 

 leaf-shape. Evidently the cytoplasms provided by the two species 



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