ai)justmi;nt and haiance 



roots of another. Or we can put a skin of one round a core of the 

 other, as in Crataegoniespilus, the graft hybrid of the hawthorn and 

 the medlar. In this way, too, as we saw, the hcxaploid and diploid 

 species of Solamim may be combined to give results whose appear- 

 ance depends on which is inside and which is outside, and on how 

 many layers of each there are. 



These chimaeras are not always fertile, nor are they always stable. 

 An irregularity at the meristem in Cytisus adami frequently exposes 

 the yellow flowers of its Laburnum core; and less often the purple 

 flowers of the Cytisus pmptireus skin succeed in displacing the 

 mongrel colour. The chimaeras produced by somatic mutation may 

 also be of these various kinds, and they show the same characteristic 

 instabilities. Indeed many ornamental plants surprise the gardener 

 when they reveal their chimerical condition, and its origin by 

 mutation of gene or plastogene, in this way. 



The normal adjustment of cell parts, of nucleus with cytoplasm, 

 is not constant like that of cell with cell. On the contrary, it is 

 regularly changing as the cytoplasm changes; and, as we have seen, 

 it is on these changes that the processes of development and differen- 

 tiation depend. Nevertheless, the changes must follow a fixed 

 path if the outcome of differentiation is to be an organism that will 

 work, and this is secured by the subordination of the changing 

 cytoplasm to the constant nuclear genotype. If we put a nucleus 

 into a cytoplasm which is not of its accustomed type, one of the 

 two has to give way or the disharmony will lead to a failure of the 

 normal sequence of changes and so to early death, as we saw in 

 the various merogons with the cytoplasm of one species and the 

 nucleus of another. When one gives way it always has to be the 

 cytoplasm: the nucleus forces the cytoplasm into its own pattern, 

 as in the Acetabtdaria grafts. 



Disharmony of nucleus and cytoplasm can be produced by 

 hybridization as well as by grafting. When made one way, a species 

 cross in Epilobium or Streptocarpus may be successful in giving a 

 hybrid capable of adequate development. Yet when made the other 

 way the hybrid may be abnormal in greater or less degree. The 

 hybrid nucleus can work with cytoplasm, or more precisely with 

 the plasmagenes, from one parent, but not with those from the 

 other. Such disharmonv is not, however, the usual cause of abnor- 



22S 



