Lastly, I have heard from Dr. E. Bornet, of Antibes, who has made numerous experiments in crossing the species of Cistus, but as not yet published the results, that, when any of these hybrids are fertile, they may be said to be, in regard to function, dioecious; "for the flowers are always sterile when the pistil is fertilised by pollen taken from the same flower or from flowers on the same plant. But they are often fertile if pollen be employed from a distinct individual of the same hybrid nature, or from a hybrid made by a reciprocal cross."
_Conclusion._--The facts just given, which show that certain plants are self-sterile, although both sexual elements are in a fit state for reproduction when united with distinct individuals of the same or other species, appear at first sight opposed to all analogy. The sexual elements of the same flower have become, as already remarked, differentiated in relation to each other, almost like those of two distinct species.
With respect to the species which, whilst living under their natural conditions, have their reproductive organs in this peculiar state, we may conclude that it has been naturally acquired for the sake of effectually preventing self-fertilisation. The case is closely analous with dimorphic and trimorphic plants, which can be fully fertilised only by plants belong to the opposite form, and not, as in the foregoing cases, in differently by any other plant. Some of these dimorphic plants are completely sterile with pollen taken from the same plant or from the same { 141} form. It is interesting to observe the graduated series from plants which, when fertilised by their own pollen, yield the full number of seed, but with the seedlings a little dwarfed in stature--to plants which when self-fertilised yield few seeds--to those with yield none--and, lastly, to those in which the plant's own pollen and stigma act on each other like poison. This peculiar state of the reproductive organs, when occurring in certain individuals alone, is evidently abnormal; and as it chiefly affects exotic plants, or indigenous plants cultivated in pots, we may attribute it to some change in the conditions of life, acting on the plants themselves or on their parents. The self-impotent _Passiflora alata_, which recovered its self-fertility after having been grafted on a distinct stock, shows how small a change is sufficient to act powerfully on the reproductive system. The possibility of a plant becoming under culture self-impotent is interesting as throwing light on the occurrence of this same condition in natural species. A cultivated plant in this state generally remains so during its whole life; and from this fact we may infer that the state is probably congenital.