DIMORPHISM AND TRIMORPHISM 310 



RECIPROCAL DIMORPHISM AND TRIMORPHISM 



This subject may be here briefly discussed, and will be 

 found to throw some light on hybridism. Several plants be- 

 longing to distinct orders present two forms, which exist 

 in about equal numbers and which differ in no respect ex- 

 cept in their reproductive organs; one form having a long 

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

 stamens ; the two having differently sized pollen-grains. 

 With trimorphic plants there are three forms likewise differ- 

 ing 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 sta- 

 mens, the three forms possess altogether six sets of stamens 

 and three kinds of pistils. These organs are so proportioned 

 in length to each other, 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 six 

 unions are legitimate, or fully fertile, — and twelve are ille- 

 gitimate, 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 



