SEXUAL AFFINITY. 78 1 



hybrids: this expresses itself only in the success of hybridisation, and may be de- 

 signated, according to Naegeli, sexual affinity. The sexual affinity does not always 

 go parallel with the external similarity of the plants ; for instance, no one has yet 

 succeeded in producing hybrids of Apple and Pear, of Anagal/is arvensis and A. 

 cartdea, of Primula officinalis and P. elatwr, of Nigella damascena and N. saliva, and 

 other systematically very similar species of the same genus, while in other cases very 

 dissimilar forms unite; for example, Acgilops ovala with Triticum vulgare, Lychnis 

 diurna withL.ßos cuculi,Cereus speciosissimus and Phyllocacius Phyllanthus , Nectarine 

 and Almond. The difference of sexual affinity and systematic alliance is still more 

 strikingly demonstrated by the fact that sometimes the varieties of the same species 

 are mutually sterile in whole or part, e. g. Silcne inflata, var. alpina with var. angusii- 

 folia ; var. lalifolia with var. litoralis ; &c. 



(3) When a sexual union of two species A and B is possible, it usually happens that 

 A can yield hybrids with pollen from B, as well as B with pollen from A (reciprocal 

 hybridisation); there are also cases however where the species A can only be the father, 

 and the species B only the mother, the pollination oi A with the pollen of -5 remaining 

 without result. Thus Thuret found that, as already stated, oospheres oi Fucus vesicu- 

 losus yield hybrids with the zoosperms of F. serratus, whereas the mixture of the 

 oospheres of F. serratus with the zoosperms of F. vesiculosus remains without 

 result. According to Q'-kxintx NicotiaJia paniculata is very prone to form h\'brid seeds 

 with the pollen of N. Langsdorfii, whereas N. Langsdorfii forms no seed with the 

 pollen of N. paniculaia. Kölreuter was able to obtain seeds easily from Mirabilis 

 Jalappa with the pollen of M. longiflora, but more than two hundred pollinations of 

 M. longiflora by M. Jalappa during eight years remained without result. 



(4) The sexual affinity presents the most various degrees. The one extreme 

 lies in the complete failure of pollination with the pollen of another variety or species, 

 so that the pollen-tubes do not even enter the stigma, and the pollinated flower be- 

 haves as if no pollen had reached it : the other extreme shows itself in the formation 

 of numerous hybrids which not only develope vigorously, but also reproduce themselves 

 sexually. Between the two extremes the most various gradations and transitions 

 occur. The lowest stages of the influence of pollen of another kind consist in that 

 various alterations take place only in the floral parts of the mother-plant itself, the 

 ovaries or these and the ovules growing, but without an embryo being developed. A 

 higher stage of the influence evinces itself in the development of ripe normal fruits 

 with seeds containing embryos, but the embryos are incapable of germinating; a 

 further advance then appears with reference to the number of ripe embryos capable 

 of development in the pollinated ovaries (cf. Hildebrand, ' Bastardirungsversuchc an 

 Orchideen! — Bot. Zeit. 1865, No. 31). 



(5) When different kinds of pollen are simultaneously transferred to the same 

 stigma, only one kind effects fertilisation, that to which the greatest sexual affinity 

 may be ascribed. Now since in general pollen acts most favourably on the fertilisa- 

 tion of another flower of the same species — in other words, since the sexual affinity 

 attains a maximum between the flowers or individuals of the same species — when 

 pollination occurs simultaneously with pollen of the same and of another species, only 

 the former is eff'ective in fertilisation ; since on the other hand hybridisation between 

 varieties is sometimes more effectual than the fertilisation of a variety by means of its 



