









ItEVTSION OF THE BRITISH WILLOWS 



341 













































sibility of hybridization, though such had been suggested ; for 

 Smith writes of " the gratuitous suppositions of the mixture of 

 species, or the production of new or hybrid ones." 



At a somewhat later period, however, the probability of the 

 phenomenon was admitted ; but it was left to Max Wichura * to 

 prove, by experiment, the truth of what had before been only 

 though on good grounds — suspected. Wichura found that not 

 only did many binary hybrids occur naturally, but that ternary 

 hybrids were also spontaneously produced. Binary hybrids are 

 those produced by two species ; ternary are those into whose 

 composition three species hare entered. In addition to this, he 

 showed that by cross-fertilizing these hybrids, plants could be 

 produced whose pedigree included no less than six species. 

 (The accompanying diagram (Plate X.) shows graphically the 

 pedigree of the compound hybrids.) Six species appeared to be 

 the limit, as, from the imperfection of the pollen or of the seeds, 

 the combination of species could not be carried further. 



Theoretically, therefore, every species can form spontaneous 

 hybrids with every other species; but practically hybrids are 

 restricted by several causes. For hybrids to occur, not only 

 niust there be a certain degree of proximity of situation in the 

 parents, but an identity in the period of flowering- Close 

 proximity, though favourable for hybridization, is not absolutely 

 necessary, since fertilization is accomplished by insect agency, 

 and the attraction to insects of willow-flowers is very great. 

 Identity of the usual time of flowering is an almost imperative 

 necessity for the production of hybrids ; but these also occur- 

 though more rarely — between species which do not ordinarily 

 flower at the same time, and must have been produced by some 

 ^normality in the period of one of the parents. It must also 

 be remembered that species, whose distribution is both lowland 

 and alpine, have their time of flowering retarded in alpine situa- 

 tions, and are hence able to hybridize with the true mountain 

 species. 



A hybrid in its best condition is exactly intermediate in cha- 

 racter between its two parents ; but more frequently it shows a 

 greater relationship with one rather than the other ; and in 

 'nose cases where it occurs in any abundance, a series of speci- 



* ' Die Bastardbefructung im Pflanzenreich erlautert an den Bastarden der 

 Weiden,' 1865. 



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