18 LAWS GOVERNING THE STEKILITY [Chap. IX. 



sume that no one will suppose that this capacity is a 

 specially endowed quality, but will admit that it is inci- 

 dental on differences in the laws of growth of the two 

 plants. We can sometimes see the reason why one tree 

 will not take on another, from differences in their rate of 

 growth, in the hardness of their wood, in the period of 

 the flow or nature of their sap, &e.; but in a multitudo 

 of cases we can assign no reason whatever. Great di- 

 versity in the size of two plants, one being woody and 

 the other herbaceous, one being evergreen and the other 

 deciduous, and adaptation to widely different climates, 

 do not always prevent the two grafting together. As in 

 hybridisation, so with grafting, the capacity is limited by 

 systematic affinity, for no one has been able to graft 

 together trees belonging to quite distinct families; and, 

 on the other hand, closely allied species, and varieties 

 of the same species, can usually, but not invariably, be 

 grafted with ease. But this capacity, as in hybridisation, 

 is by no means absolutely governed by systematic 

 affinity. Although many distinct genera within the 

 same family have been grafted together, in other cases 

 species of the same genus will not take on each other. 

 The pear can be grafted far more readily on the quince, 

 which is ranked as a distinct genus, than on the ap- 

 ple, which is a member of the same genus. Even dif- 

 ferent varieties of the pear take with different de- 

 grees of facility on the quince; so do different varie- 

 ties of the apricot and peach on certain varieties of the 

 plum. 



As Gartner found that there was sometimes an 

 innate difference in different individuals of the same 

 two species in crossing; so Sageret believes this to be 

 the case with different individuals of the same two spe- 



