Pollen may be transferred to the stigma of the same flower - self-pollination (or autogamy), or to the stigma of another flower on the same plant or another plant of the same species - crosspollination (or allogamy).
In bisexual or hermaphrodite flowers, that is, those in which both stamens and pistil are present, though self-pollination might seem the obvious course, this is often prevented or hindered by various arrangements which favour cross-pollination.
For instance, in many flowers the filaments are at first directed outwards so that self-pollination is not possible, but later incline towards the stigmas and pollinate them (e.g.
Spontaneous self-pollination is rendered impossible in some homogamous flowers in consequence of the relative position of the anthers and stigma - this condition has been termed herkogamy.
Flowers in which the relative position of the organs allows of spontaneous self-pollination may be all alike as regards length of style and stamens (homomorphy or homostyly), or differ in this respect (heteromorphy) the styles (From Strasburger's by permission of Gustav Fischer.) FIG.
Has ceased to be receptive before the anthers open, or the anthers have withered before the stigma becomes receptive, when crosspollination only is possible, or the stages of maturity in the two organs are not so distinct, when self-pollination becomes possible later on.
Arrangement self-pollination is prevented and cross-pollination ensured by the visits of bees which come for the honey secreted by the glands at the base of the inner stamens.