280 RECIPROCAL DIMORPHISM 



observers, that in order to obtain full fertility with these 

 plants, it is necessary that the stigma of the one form should 

 be fertilized by pollen taken from the stamens of correspond- 

 ing 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 six unions are legiti- 

 mate, or fully fertile, and twelve are illegitimate, or more 

 or less infertile. 



The infertility which may be observed in various dimor- 

 phic and trimorphic plants, when they are illegitimately 

 fertilized, that is, by pollen taken from stamens not corre- 

 sponding 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 favorable, so I 

 have found it with illegitimate unions. It is well knowu 

 that if pollen of a distinct species be placed on the stigma 

 of a flower, and its own pollen be afterward, 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 fertilizing 

 several flowers, first illegitimately and twenty-four hours 

 afterward legitimately, with pollen taken from a peculiarly 

 colored variety, and all the seedlings were similarly colored ; 

 this shows that the legitimate pollen, though applied twenty- 

 four hours subsequently, had wholly destroyed or pre- 

 vented 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 fertilized 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 fertilized 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 illegiti- 

 mately united, behave in exactly the same manner as do two 

 distinct species when crossed. This led me carefully to. 



