WEISMANN'S THEORY OF THE GERMPLASM 69 



One would expect, therefore, that a twig and 

 stem, chosen from two closely allied species, such 

 as, for instance, the pear and the apple, would 

 unite when the suitable tissues were put together. 

 But this does not happen. Successful grafting 

 depends far less on the conjunction of obviously 

 appropriate parts than upon characters unrecognis- 

 able by us, such as deep-seated kinship between 

 the parts, and the specific characters of their cells ; 

 while in the case of individuals of the same species 

 two pieces will unite even if they are not brought 

 together in appropriate conjunction, or when they 

 belong to different parts of the organism, as, for 

 instance, to the root and the leaf; yet in the 

 absence of deep-seated kinship union will not take 

 place. 



Generally this kinship, which has been called 

 vegetative affinity, depends, like sexual affinity, 

 upon the degree of systematic relationship. It 

 appears that the same condition of things occurs 

 as when, in ordinary fertilisation, sexual cells from 

 different varieties, or species, are united. In both 

 cases it happens, on the average, that union is the 

 more to be expected the more closely the plants 

 concerned are akiu, in a natural system of classifica- 

 tion. 



But in grafting, as in cross-fertilisation, unex- 

 pected exceptions to this rule occur. Relying upon 

 these, Naegeli thought that the external distinguish- 

 ing tokens do not always indicate correctly the 

 intrinsic constitutional differences. Frequently 



