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Show 266 CONCLUDING REMARKS CHAP. VI. the anthers in size, and the pollen-grains in diameter. It appears, therefore, at first sight probable that organs which differ in such important respects could act on one another only in some manner for which they had been specially ada pte d. The pro ba hili ty of this view is supported by the curious rule that the greater the difference in length between the pistils and stamens of the trimorphic species of Lythrum and Oxalis, the products of which are united for reproduction, by so much the greater is the infertility of the union. The same rule applies to the two illegitimate unions of so1ne dimorphic species, na1nely, Primula vulgaris and Pulmonaria angustifolia; but it entirely fails in other cases, as with Hottonia palustris and Linum grandijlor~tm. We shall, however, best perceive the difficulty of understanding the nature and origin · of the co-adaptation between the reproductive organs of the two forms of heterosty led plants, by considering the case of. Linum grandi.florum : the two forms of this plant differ exclusively, as far as we can see, in the length of their pistils ; in the long-sty led form, the stamens equal the pistil in length, but their pollen has no more effect on it than so much inorganic dust; whilst this pollen fully fertilises the short pistil of the other fonn. Now, it is scarcely credible that a n1ere difference in the length of the pistil can make a wide difference in its capacity for being fertilised. We can believe this the less because with some plants, for instance, Amsinckia spectabilis, the pistil varies greatly in length without affecting the fertility of the individuals which are intercrossed. So again I observed that the same plants of Primula veris and vulgaris differed to an extraordinary degree in the length of their pistils puring successive seasons ; nevertheless they yielded during these seasons exactly CHAP. VI. ON HETEROSTYLED PLANTS. 267 the same average number of seeds when left to fertilise themselves spontaneously under a net. We must therefore look to the appearance of inner or hidden constitutional differences between the individuals of a varying species, of such a nature that the male element of one set is enabled to act efficiently only on the female ele1nent of another set. We need not doubt about the possibility of variations in the constitution of the reproductive system of a plant, for we know that some species vary so as to be completely self-sterile or completely self-fertile, either in an apparently spontaneous 1nanner or fro1n slightly changed conditions of life. Gartner also has shown* that the individual plants of the same species vary in their sexual powers in such a manner that one will unite with a distinct species much more readily than another. But what the nature of the inner constitutional differences may be between the sets or forms of the same varying species, or between distinct species, is quite unknown. It seems therefore probable that the species which have become heterostyled at first varied so that two or three sets of individuals were formed differing in the length of their pistils and stamens and in other co-adapted characters, and that almost simultaneously their reproductive powers became modified in such a manner that the sexual elements in one set were adapted to act on the sexual elements of another set; and consequently that these elements in the same set or fonn incidentally became ill-adapted for mutual interaction, as in the case of distinct species. I have elsewhere shown t that the sterility of species when . * Gartner, ' Bastarderzeugung 1m P,flan.z~nreich,' 1849, p. 165. t Or1gm of Species,' tith edit. P· 24 7 ; ' Variation of Animals and Plants under Domestication,' 2nd edit. vol. ii. p. 169; 'The Effects of Cross and Self-fertilisation,' p. 463. It may be well here to remark |