REPRODUCTION. 65 1 



for determining the relation of the hybrid to its two parents. 

 If hybrids, namely, are fertilised through several generations 

 by pollen from one or other of the parent forms, the progeny, 

 the derivative hybrids as they are called, gradually reassume 

 the parent form. If then a hybrid be precisely intermediate 

 in character between its two parents, it will require just the 

 same number of successive fertilisations by the pollen of each 

 parent form to produce derivative hybrids which exactly 

 resemble each parent form. If, on the other hand, the hybrid 

 partakes more of the nature of one parent than of the other, 

 the number of fertilisations necessary to reproduce the one 

 parent form will be less than that necessary to reproduce the 

 other. For instance, Gartner observed that when the hybrid 

 of Dianthus chinensis and of D. Caryophyllus was fertilised in 

 successive generations by the pollen of D. Caryophylhis, the de- 

 rivative hybrid of the third or fourth generation was a D. Caryo- 

 phyllus ; whereas when fertilised by the pollen of D. chinensis, 

 it was not until the fifth or sixth generation that the D. chinen- 

 sis was reproduced. In this case D. Caryophyllus was clearly 

 prepotent over D. chinensis in the production of the hybrid. 



There is a fact of great interest, which may be conveni- 

 ently mentioned here, that hybrids may be produced not 

 only by means of sexual, but also vegetative reproduction. 

 Hybrids produced in this way are termed graft-hybrids. As 

 a rule, in the process of grafting neither the graft (or scion} 

 or the stock is affected ; each, as it grows, manifests in all its 

 organs its own characteristics. But instances are on record 

 of their mutually affecting each other. A well-known case of 

 this kind is that of the Cytisus Adami. The origin of this 

 form is stated as follows : a shoot of Cytisus purpureus was 

 grafted on a stock of Cytisus Laburnum : from this were 

 produced many shoots, one of which grew vigorously, and 

 developed larger leaves than those of C. purptireus, and from 

 this shoot plants were propagated constituting the Cytisus 

 Adami. On flowering, it was found that the flowers were of 

 a dingy red. Various other cases of the same kind, notably 

 of the effect produced on the stock by grafting scions with 

 variegated leaves, are given by Darwin, 



