30 RECIPROCAL DIMORPHISM [Chap. IS, 



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 il- 

 legitimately fertilised, that is by pollen taken from 

 stamens not corresponding in height with the pistil, 

 differs much in degree, np 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 in an eminent degree on the con- 

 ditions 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 after 

 a considerable interval of time, placed on the same 

 stigma, its action is so strongly prepotent that it 

 generally anniliilates the effect of the foreign pollen ; 

 so it is with the pollen of the several forms of the same 

 species, for legitimate pollen is strongly prepotent over 

 illegitimate pollen, when both are placed on the same 

 stigma. I ascertained tliis by fertilising several flowers, 

 first illegitimately, and twenty-four hours afterwards 

 legitimately, with pollen taken from a peculiarly 

 coloured variety, and all the seedlings were similarly 

 coloured ; this shows that the legitimate pollen, though 

 applied twenty-four hours subsequently, had wholly 

 destroyed or prevented the action of the previously 

 applied illegitimate pollen. Again, as in making re- 

 ciprocal crosses between the same two species, there is 

 occasionally a great difference in the result, so the same 

 thing occurs with trimorphic plants ; for instance, the 

 mid-styled form of Lythrum salicaria was illegitimately 

 fertilised with the greatest ease by pollen from the 



