Page - 319 - in The Origin of Species

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