MEANS OF FERTILISATION.
Sterility and fertility of plants when insects are
The means by which flowers are cross-fertilised.
Structures favourable to self-fertilisation.
Relation between the structure and conspicuousness of flowers, the
visits of insects, and the advantages of cross-fertilisation.
The means by which flowers are fertilised with pollen from a distinct
Greater fertilising power of such pollen.
Conversion of anemophilous species into entomophilous.
Origin of nectar.
Anemophilous plants generally have their sexes separated.
Conversion of diclinous into hermaphrodite flowers.
Trees often have their sexes separated.
In the introductory chapter I briefly specified the various means by which cross-fertilisation is favoured or ensured, namely, the separation of the sexes,—the maturity of the male and female sexual elements at different periods,—the heterostyled or dimorphic and trimorphic condition of certain plants,—many mechanical contrivances,—the more or less complete inefficiency of a flower’s own pollen on the stigma,—and the prepotency of pollen from any other individual over that from the same plant. Some of these points require further consideration; but for full details I must refer the reader to the several excellent works mentioned in the introduction. I will in the first place give two lists: the first, of plants which are either quite sterile or produce less than about half the full complement of seeds, when insects are excluded; and a second list of plants which, when thus treated, are fully fertile or produce at least half the full complement of seeds. These lists have been compiled from the several previous tables, with some additional cases from my own observations and those of others. The species are arranged nearly in the order followed by Lindley in his ’Vegetable Kingdom.’ The reader should observe that the sterility or fertility of the plants in these two lists depends on two wholly distinct causes; namely, the absence or presence of the proper means by which pollen is applied to the stigma, and its less or greater efficiency when thus applied. As it is obvious that with plants in which the sexes are separate, pollen must be carried by some means from flower to