Chap. 1X.~_ AND TRIMORPHISM. 253 



that half the stamens in two of the forms 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 cor- 

 responding height in another form. 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 sis 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 in 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 after a considerable interval of time, 

 placed on the same stigma, its action is so strongly prepotent that 

 it generally annihilates 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 this by fertilising several 

 flowers, first illegitimately, and twenty-four hours afterwards legiti- 

 mately, 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 reciprocal 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 longer stamens 

 of the short-styled form, and yielded many seeds ; but the latter 

 form did not yield a single seed when fertilised by the longer 

 stamens of the mid-styled form. 



In all these respects, and in others which might be added, the 

 forms of the same undoubted species when illegitimately united 

 behave in exactly the same manner as do two distinct species when 

 crossed. This led me carefully to observe during four years many 

 seedlings, raised from several illegitimate unions. The chief result is 



