It will be convenient at this stage to refer briefly to the two principal methods by which the pollen of flowers is transferred to the stigmas for the purposes of fertilisation, and to see how various species are structurally adapted to the means by which the transfer is brought about.
Speaking generally, we may classify flowers into those which are fertilised by the wind (anemophilous flowers) and those in which the pollen is transferred by insects (entomophilous flowers). The former offer no attractions to allure the various forms of insect life. They are, generally speaking, very inconspicuous, being of small size and having no bright corollas. None of them are scented, nor do they produce the sweet nectar that forms the principal food of so many insects. Their anthers are borne on long filaments, so that they are exposed freely to the wind; and they produce abundance of pollen to compensate for the very wasteful method of wind-dispersion. The pollen, too, is not very adherent, so that it may be readily carried away by the breeze; and the plants concerned often produce their flowers early in the spring, before the leaves have appeared, thus giving the wind very free play.
Fig. 277.—Expanded Spikelet of the Oat
G. glumes; P.e, outer pale; P.i, inner pale; A, awn; F.S, a sterile flower. The stamens and the feathery stigmas of the fertile flower are also shown
Insect-fertilised flowers, on the other hand, are usually of attractive appearance; and, though often small and inconspicuous individually, they are in such cases grouped together in more or less showy clusters. They are also usually scented, and supply nectar and pollen to the insects which they allure. Some are fertilised by insects that fly by day, and these often close their petals on the approach of night, thus protecting their pollen during the period in which their fertilisers sleep. Others, fertilised by nocturnal insects, always spread their petals during the night, and generally protect their pollen from waste by sleeping throughout the day. As a rule, too, these night-bloomers have large and pale-coloured petals that are more easily seen by night; they also evolve a powerful scent to aid the insects in searching them out.
It will be seen that the economic relationship existing between flowers and insects is a mutual one, the latter visiting the former in order to obtain food, while the former derive in return the advantage of a direct transfer of pollen from flower to flower.
It is a well-known fact that the self-fertilisation of a flower often results in the development of very weak seedlings as compared with those that are produced by crossing; and it often happens that the pollen of a flower is incapable of producing the least effect when deposited on the stigma of the same bloom. In some cases the contact of the pollen of a flower with its own stigma will even act as a poison, causing the whole to shrivel and die; and truly wonderful are the varied means by which flowers contrive to secure a cross-fertilisation. It is here that the work of the wind and insects proves so valuable to flowers; but, in addition to this, a very large number of flowers are absolutely incapable of self-fertilisation, for the anthers and the stigma are not mature at the same time, or they exist in separate flowers, either on the same plant or on distinct plants of the same species. It is most interesting and instructive to study the many contrivances by which flowers compel certain insects to convey the pollen exactly in the way that best serves their purpose, sometimes even entrapping them after they have been allured, and not allowing them to escape until they are thoroughly dusted with the pollen which they are required to convey; but it is hardly our province to enter more fully into this matter in these pages.
An examination of the grasses will show at once that they are adapted for fertilisation by the wind. The flowers produce no nectar; and, consistently, develop no bright petals and evolve no odours to attract insects. On the other hand, their anthers produce abundance of lightly-adhering pollen, and are mounted on long filaments which hold them well exposed to the wind; and the stigmas are well adapted for catching the scattered grains, being long and protruding, and often covered with sticky hairy or feathery appendages.