THE CYTOPLASM 



on which way the cross is made, thus (putting the female parent 

 first): — 



yellow X green > yellow 



green X yellow >- green 



Evidently this inheritance is non-mcndelian and therefore non- 

 nuclear. The effective agent must be in the cytoplasm, though 

 whether it is attached to the plastids or is free from them and, like 

 the gene in the other case, acting on them, we cannot yet say. 



Crosses between species of Oenothera answer this question. Two 

 green species when crossed together sometimes give yellow off- 

 spring — but only one way. There are always differences between 

 the reciprocal crosses. In Reimer's experiments, for example: — 



Oe. hookeri X muricata {curvans) > yellow 



Oe. muricata [curvans) X hookeri > green 



Thus the plastids o£ hookeri, which are green in the parent species, 

 are yellow with the nucleus of the hybrid. But the plastids of 

 muricata remain green in the hybrid. Now the nuclei in the 

 reciprocal hybrids must be the same. It must, therefore, be the 

 plastids or the cytoplasms that differ. One of them gets on with its 

 parental nucleus but fails to get on with that of the hybrid. Thus 

 both the nucleus and certain agents outside it play a part in the 

 reaction. 



This is not all. The yellow F^ seedlings die but the green seedlings 

 from the muricata mother occasionally develop flakes of yellow in 

 their shoots. These are evidently derived from odd hookeri plastids 

 which have come in with the pollen and sorted themselves out 

 during development to make the full plastid complements of certain 

 cells. Now, these yellow flakes can form the sub-epidermis of a 

 shoot; they then produce the germ cells and can be used for breeding. 

 When sclfed, such a shoot gives two kinds of offspring; there 

 is a simple segregation of the hookeri-muricata [curvans) nuclear 

 difference, to give two types: one homozygous hookeri and the 

 other heterozygous like the F^ hybrids; the homozygous muricata 

 [curvans), dies in embryo. The two kinds are alike in having hookeri 

 plastids and differ only in the nucleus. The hcterozygote is again 

 yellow; but the homozygote is green. Thus the hookeri plastids 



170 



