Chap. XIX, HYBRIDISM. 181 



reciprocally dimorphic and trimorphic plants, when illegiti- 

 mately united. I have had occasion several times to allude to 

 these plants, and I may here give a brief abstract 2 of my 

 observations. Several plants belonging to distinct orders pre- 

 sent two forms, which exist in about equal numbers, and which 

 differ in no respect except in their reproductive organs; one 

 form having a long pistil with short stamens, the other a short 

 pistil with long stamens ; both with differently sized pollen- 

 grains. With trimorphic plants there are three forms likewise 

 differing in the lengths of their pistils and stamens, in the size 

 and colour of the pollen-grains, and in some other respects ; and 

 as in each of the three forms there are two sets of stamens, 

 there are altogether six sets of stamens and three kinds of 

 pistils. These organs are so proportioned in length to each other 

 that, in any two of the forms, half the stamens in each stand 

 on a level with the stigma of the third form. Now I have shown, 

 and the result has been confirmed by other observers, that, in 

 order to obtain full fertility with these plants, it is necessary 

 that the stigma of the one form should be fertilised by pollen 

 taken from the stamens of corresponding height in the other 

 iorm. So that with dimorphic species two unions, which may 

 be called legitimate, are fully fertile, and two, which may be 

 called illegitimate, are more or less infertile. With trimorphic 

 species six unions are legitimate or fully fertile, and twelve are 

 illegitimate, or more or less infertile. 



The infertility which may be observed in various dimorphic 

 and trimorphic plants, when they are illegitimately fertilised, 

 that is, by pollen taken from stamens not corresponding in height 

 with the pistil, differs much in degree, up to absolute and utter 

 sterility ; just in the same manner as occurs in crossing distinct 

 species. As the degree of sterility in the latter case depends 

 m an eminent degree on the conditions of life being more or 

 less favourable, so I have found it with illegitimate unions. It 

 is well known that if pollen of a distinct species be placed on 

 the stigma of a flower, and its own pollen be afterwards, even 



2 This abstract was published in the my original observations on this point 



fourth edition (1866) of my < Origin of have not as yet been published in 



Species; ' but as this edition will be in detail, I have ventured here to reprint 



the hands of but few persons, and as the abstract. 



