CHAPTER X.. Means of Fertilisation.

MEANS OF FERTILISATION.

Sterility and fertility of plants when insects are excluded.--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 plant.--Greater fertilising power of such pollen.--Anemophilous species.--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.