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Show 244 CONCLUDING REMARKS CHAP. VI. CHAPTER VI. CoNCLUDING REMARKS oN HETEROSTYLED PLANTS. The essential character of heterostyled plants-Summary of the ·differences in fertility between legitimately and illegitimately fertilised plants-Diameter of the pollen-grains, si~e. of anthers anti strm~ture of stigma in the different forms-Affimtles of tho genera which include heterostyled species-Nature of the advantages derived from heterostylism-The means by which plants became heterosty1ed-Transmission of form-Equal-styled varieties of l1eterostyled plants-Final remarks. IN the foregoing chapters all the heterostyled plants known to me have been more or less fully described. Several other ·cases have been indicated, especially by Professor Asa Gray and Kuhn,* in which the individuals of the same species differ in the length of their stamens and pistils; but as I have been often deceived by this character taken alone, it seems ~o me the more prudent course not to rank any spe~ws as heterostyled, unless we have evidence of more. 1mpor· tant differences between the forms, as in the diameter of the pollen -grains, or in the structure of th.e stigma. The individuals of many ordinary hermaphrocht~ plants , habitually fertilise one another, owing to theu ~ale and female organs being mature at clifferen~ yenods: or to the structure of the parts, or to self-stenhty, &c.' and so it is with many hermaphrodite animals, for instance, land-snails or earth-worms ; but in all thes.e cases any one individual can fully fertilise or be ferti· * Asa Gray., ~ Amedcan J ourn. elsewhere as already referred ;~· of Science,' 18n.5, p. 101 ; and Kuhn, ' Bot. Zeitung,' 1867, P· · CHAP. VI. ON HETEROSTYLED PLANTS. 245 lised by any other individual of the same species. This is not so with heterostyled plants: a long-styled, midstyled or short-styled plant cannot fully fertilise or be fertilised by any other individual, but only by one belonging to another form. Thus the essential character of plants belonging to the heterostyled class is that the individuals are divided into two or three bodies, like the males and females of dimcious plants or of the higher animals, which exist in approximately equal numbers and are adapted for reciprocal fertilisation. The existence, therefore, of two or three bodies of individuals, differing from one another in the above more important characteristics, offers by itself good evidence that the species is heterostyled. But absolutely conclusive evidence can be derived only from experiments, and by :finding that pollen must be applied from the one form to the other in order to ensure complete fertility. In order to show how much more fertile each form is when legitimately fertilised with pollen from the other form (or in the case of trimorphic species, with the proper pollen from one of the two other forms) than when illegitimately fertilised with its own-form pollen, I will append a Table (33) giving a summary of the results in all the cases hitherto ascertained. The fert~lity of the unions may be judged by two standards, namely, by the proportion of flowers which, when fertilised in the two methods, yield capsules, and by the average number of seeds per capsule. When there is a dash in the left-hand column opposite to the name of the species, the proportion of the flowers which yielded capsules was not recorded. The two or three forms of the same heterosty lefl speci~s ·do not differ from one another in general habit or fohage, as sometimes, though rarely, happens with |