30 RECIPROCAL DIMORPHISM [Chap. IX. 



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, up to absolute and utter steril- 

 ity; just in the same manner as occurs in crossing dis- 

 tinct 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 anni- 

 hilates 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 il- 

 legitimately, and twenty-four hours afterwards legiti- 

 mately with the 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 il- 

 legitimate pollen. Again, as in making reciprocal 

 crosses between the same two species, there is occasion- 

 ally a great difference in the result, so the same thing 

 occurs with trimorphic plants; for instance, the mid- 

 styled form of Lythrum saliearia was illegitimately fer- 

 tilised 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 



