HVBRIDISA TION. 



819 



of numerous hybrids, which not only grow vigorously, but are themselves fertile. 

 The lowest degree of the action of pollen of a different kind consists in various 

 changes taking place in the parts of the flower of the mother-plant, the ovary or even 

 the ovules also growing, without any embryo being produced. A higher degree is 

 manifested in the production of ripe normal fruits and seeds containing embryos, 

 but these embryos having no power of germination. Further steps are indicated by 

 the number of embryos which have the power of germination that are produced in the 

 ovary \ 



When pollen from different species is applied simultaneously to the same 

 stigma, only one kind is potent, viz. that from the species which has the greatest 

 sexual affinity to the one that is pollinated. And since, as a general law, pollen is most 

 efficacious on a different flower of the same species — in other words, the highest 

 degree of sexual affinity occurs between different individuals of the same species — 

 when a stigma is pollinated at the same time with pollen of the same and of another 

 species, the first only is potent. But since, on the other hand, hybrids are sometimes 

 more easily produced between varieties than between individuals of the same variety, 

 in this case the foreign pollen may be prepotent over that of the same kind. When 

 the pollen of different species reaches the stigma at the same time, and if that which 

 reaches it later has a greater sexual affinity, it can only be potent when the first is 

 not potent or acts injuriously. In Nicotiana the production of hybrids can no longer 

 be prevented by its own pollen after two hours, in Malva and Hibiscus after three 

 hours, in Dianthus after five or six hours. 



The hybrid is possessed of external characters intermediate between those of its 

 parent-forms, usually nearly half way between ; less often it resembles one of the 

 parent-forms more nearly than the other, and this is more often the case with variety- 

 hybrids than with species-hybrids. It follows that in reciprocal hybrids from the 

 species A and B, the hybrid A B is generally similar externally to the hybrid B A, 

 though the two forms may differ somewhat internally. Thus, according to Gartner, 

 the hybrid Nicotiana paniculato-rustica is more fertile than the reciprocal hybrid 

 Nicotiana riistico-panicidata'^. An internal difference between reciprocal hybrids is 

 also show^n by the fact that one is more variable than the other ; thus, according to 

 Gartner, the progeny oi Digitalis purpureo-liitea is more variable than that oi D.lu'co- 

 piirpiirea^ the progeny of Dianthus pidchello-arenarius more variable than that of 

 D. arenario-pulchellns. 



When two species A and B hybridise, and the one species A exercises a greater 

 influence on the form and properties of the hybrid than the other species B, the 

 hybrid or its descendants, if fertilised by A, will revert more quickly to the parent- 

 form A than it will to the parent form B if fertilised by it. Thus Gartner states that 

 the hybrid of Dianthus chinensis and D. Caryophytltis reverts to the latter form after 

 three or four generations if repeatedly fertilised by it, while it requires fertilisation for 

 five or six generations by D. chinensis in order to revert to that form. 



^ See Hildebrand, Bastaidirungsversuche an Orchideen, Bot. Zeit. 1865, No. 31. 



2 In this mode of designating hybrids, the name of the male parent-plant stands first ; thus 

 Nicotiana riistico-paniculata is the product of the fertilisation of N. pafiiaJata by the pollen of 

 N. rui>(ica. 



3 G 2 



