CHAPTER 8 



THE CYTOPLASM 



Plastogeties Plasmagenes Cytoplasmic Equilibrium Cytoplasm, 



Nucleus and Etwironment Dauermodification 



Suppressiveness and Amhilinearity 



The cytoplasm must be the agent of the nucleus in development 

 for it is only from the cytoplasm that the nucleus can obtain its 

 raw materials, and it is only to the cytoplasm that it can pass on the 

 products of gene action. But the cytoplasm might also act as an 

 agent in its own right, by virtue of its own capacities of self- 

 propagation. Clearly the way of testing this possibility is by bringing 

 together cytoplasm and nucleus in new associations. 



There are two ways of effecting this re-association. The first is 

 by grafting, whose consequences we have seen in Acetahularia. The 

 second is by breeding. Sometimes the nucleus of one species can 

 be associated with the cytoplasm of another. Thus an occasional 

 seed borne on the lentil plant. Lens esculenta, in cultivation grows 

 into a common vetch, Vicia sativa. Vetch pollen has entered the 

 lentil embryo-sac. The male generative nucleus has supplanted 

 the egg nucleus, doubled its chromosomes, and developed as a 

 diploid vetch in a lentil seed coat. These intruders are entirely 

 normal and show no effect of the cytoplasm in heredity. There 

 are also instances of such exceptional behaviour in Fragaria and 

 Nicotiana. 



These rare examples do not, however, offer a sure basis for 

 generalization, as we can see if we turn to animal merogons. 

 Sometimes development is successful and, in the long run, patro- 

 clinal, as we saw earlier in sea-urchin crosses where the egg-nucleus 

 has been artificially removed before fertilization (Fig. 2). In similar 

 merogons between the newts Triton palmatns and T. cristatus the 

 outcome is less successful. These develop under the control of the 

 cytoplasm, that is maternally, for some time. But in the end they 

 become malformed and development ceases. Evidently at the 

 beginning, the cytoplasm had enough of the products of its past 



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