The transfer of pollen grains from the anther of a stamen to the receptive part of a carpel, the stigma is known as pollination. In most cases, the agency by which the pollen is transferred, is one of two kinds, by insects, or by wind. Flowers are said to be entomophilous if insects are the agents of pollination while they are said to be anemophilous if the agent of pollination is wind. Entomophilous flowers generally have the following characteristic features:
- They are large, brightly colored and often scented. If small, they are aggregated in numbers to form a broad expanse of color against the background. Insects are thus attracted visually and sometimes through the alfactory sense over some distance.
- The flowers provide a source of food for insects which imbibe sugary nectar from the nectarines or collect the pollen, or both.
- Nectaries, anthers and stigmas are situated in a special relationship to one another, so that the anthers and stigmas must be disturbed if the nectar or pollen is to be reached by the insect, i.e. there may be highly specialized pollination mechanisms in the flower, e.g early purple orchis.
- The pollen grains of entomophilous flowers are usually thick-walled, sticky and spiny to enhance adhesion to insect bodies.
By whatever agency pollen may be transported, there are two distinct possibilities with reference to its eventual destinations. First, the pollen may be transferred from an anther to a stigma in the same flower or in another flower on the same plant. Secondly, the pollen may be transferred from an anther in a flower on one plant to a stigma in a flower on another plant of the same species. The former constitutes self –pollination while the latter is referred to as cross-pollination.
If gametic union is effected between male and female gametes produced by the same plant, the mixture is less likely to lead to variation than if the gametes of different plants unite. In other words, cross-pollination is a more effective method of producing new variants than self-pollination. Hybridization by plant breeders is effected by controlled cross-pollination, in which pollen is actually transferred from one plant to another by a brush.