PLASTOGENES 



which turned yellow with the hybrid nucleus turn green again 

 when restored to their own proper nucleus. 



The sorting out of the colour determinant from mixed cells is a 

 characteristic defect of cytoplasmic transmission. Now if the extra- 

 nuclear determinant were of a molecular size it would scarcely be 

 expected to sort out so quickly. It seems likely therefore, although 

 mixed plastids have not so far been seen in the cells of variegated plants 

 that it is the plastid itself which carries the determinant of its own 

 character. This story shows that the plastids are permanent in their 

 character. It also shows that this character is adjusted in the green 

 plant to the character of the nucleus with which it has to do its 

 work. The interaction of the two is that of two complementary 

 genes, one mendelian, the other not mendehan. 



To what does a plastid owe its permanent or self-governing 

 genetic character? So far we have attributed all permanence to 

 genes, tacitly assuming that they were all nuclear genes; an 

 assumption, incidentally, which Johannsen refused to make when he 

 proposed the term. Now, it seems, we must recognize the existence 

 of determinants having the properties of genes but existing outside 

 the nucleus, and even capable of surviving their disagreement with 

 the nucleus. They are speciaHzed by their attachment to the plastids 

 and Imai has called them plastogenes. 



Since, as we saw, the plastids of different species differ in genetic 

 character, their plastogenes, like other genes, must be liable to 

 mutation. Like ordinary gene mutation it is in most plants a rare 

 event. In most Oenothera species a mutation is seen once in about 

 2,000 plants. But occasionally, as with gene mutation, it may 

 become frequent, and it is then seen to be under a genetic 

 control which is partly external to the plastid itself The 

 inheritance of certain kinds of variegation shows how this control 

 works. 



In barley, Hordeum vulgare, Imai found a variety in which the 

 plastids are characteristically unstable. They often change during 

 growth from green to white. Sorting out then gives green and 

 white cells, and later green and white variegated tissues. Most of the 

 eggs contain green plastids and most of the selfed seedlings, therefore, 

 give rise once more to variegated plants. But some eggs contain 

 only white plastids and these eggs give rise to albino seedhngs which 



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