2t2 LAWS GOVERNING THE STERILITY 



no reason whatever. Great diversity 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 hybridization, so with grafting, the capa- 

 city 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 hybridization, 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 apple, which is a member of the same 

 genus. Even different varieties of the pear take with differ- 

 ent degrees 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 differ- 

 ent individuals of the same two species in being grafted 

 together. As in reciprocal crosses, the facility of effecting 

 an union is often very far from equal, so it sometimes is in 

 grafting. The common gooseberry, for instance, cannot be 

 grafted on the currant, whereas the currant will take, though 

 with difficulty, on the gooseberry. 



We have seen that the sterility of hybrids which have 

 their reproductive organs in an imperfect condition, is a 

 different case from the difficulty of uniting two pure species 

 which have their reproductive organs perfect ; yet these two 

 distinct classes of cases run to a large extent parallel. Some- 

 thing analogous occurs in grafting; for Thouin found that 

 three species of Robinia, which seeded freely on their own 

 roots, and which could be grafted with no great difficulty on 

 a fourth species, when thus grafted were rendered barren. 

 On the other hand, certain species of Sorbus, when grafted 

 on other species, yielded twice as much fruit as when on 

 their own roots. We are reminded by this latter fact of the 

 extraordinary cases of hippeastrum, passiflora, etc., which 

 seed much mors freely when fertilized with the pollen of a 

 distinct species than when fertilized with pollen from the 

 same plaui. 



