144 



CLELAND 



these were large enough to show much structure under the ordinary micro- 

 scope and none seemed to undergo any sort of marked change or cyclical 

 modification. There seemed little, therefore, that the cytologist could learn 

 about this portion of the cell (fig. 8). 



This situation is rapidly changing, however, in view of the discoveries by 

 both the geneticists and the biochemists. The geneticists have discovered that 

 the cytoplasm has a role in heredity which is far larger than had originally 

 been suspected. The plastids, for instance, have been shown to have hereditary 



Fig. 8. A diagram of a cell. The cen- 

 tral sphere is the nucleus, the rod- 

 shaped bodies the chromosomes. The 

 region outside the nucleus is the cyto- 

 plasm, and in it are shown the more or 

 less spherical plastids and the much 

 smaller spherical or rod-shaped chon- 

 driosomes. 



characteristics of their own which to a degree are independent of those of the 

 genes. To be sure, the genes set up the conditions under which the plastids 

 operate, and if the wrong genes are present, a given kind of plastid may not 

 be able to function successfully. There are also cases where a gene may 

 succeed in bringing about a more or less permanent change in the structure 

 or function of a plastid. On the other hand, cases are known where plastids 

 remain uninfluenced by foreign genes, in whose presence they are unable to 

 function, but to whose influence they fail to yield — so that if they are re- 

 moved from the presence of the uncooperative gene and find themselves 

 again in a congenial genie environment, they can again function normally, 

 unchanged in their fundamental characteristics by having been associated 

 with the wrong kind of gene. Hence, plastids seem to have a degree of inde- 

 pendence of the gene. In the evening primroses, for instance, several geneti- 

 cally distinct classes of chloroplasts are known (Schotz, 1954). They retain 

 their own specific characteristics indefinitely, no matter what kinds of genes 

 they are exposed to. If they are subjected to the wrong kinds of genes, they 

 will be unable to function, but relieved of these genes, they will continue to 

 function as before. Each class of plastid reacts differently to the various types 

 of genes which exist in the population, and each retains its own characteristics 

 no matter what the vicissitudes to which it is subjected. On the other hand, 



