POLLINATION, a term used in botany for the transference of pollen (see FLOWER) to the stigma (the receptive surface) of the ovary of the flower. Such pollination brings about the fertilisa tion of the ovules in the ovary and their subsequent development into seeds ; there are, however, a few cases in which parthenogene sis occurs, i.e., the ovules develop without fertilisation. As the pollen-bearing parts of the stamens are rarely in contact with the stigma at the time when both of these are ripe, some mechanism is clearly necessary to bring the pollen to the stigma. The means in question is usually wind or insects, though sometimes other agencies such as water or birds may be responsible. The great variety in the form, colour and scent of flowers has been devel oped in relation to the particular agency of insects. Apart from the mechanism of pollination we can distinguish two types—self pollination (autogamy) in which pollen is transferred from the stamens of one flower to the stigma of the same flower; and cross-pollination (allogamy) in which pollen is transferred to the stigma of another flower on the same plant (geitonogamy) or to the flower of another plant of the same species (xenogamy). Occasionally hybridization is possible, the pollen of one plant bringing about fertilisation of the ovary of the flower of another species or, more rarely, of the flower of a plant belonging to an other genus; cases of hybridization between genera are known for example in cycads (see GYMNOSPERMS) and in orchids.
bearing both stamens and carpels, either self-pollination or cross pollination can occur. It is interesting to note however, that many flowers have special arrangements to ensure that the pollinating mechanism, whatever it may be, causes cross pollination and not self-pollination. One of the commonest methods to achieve this is a separation in time of the sexes—the stamens dehisce and shed their pollen either before or after the stigma is receptive. This separation in time—and it may apply to the separate male and female flowers on the same plant—is known as dichogamy. When the stamens ripen first it is known as protandry, the more com mon case, while when the stigma is ready first, it is known as proterogyny. Protandry is very common in insect-pollinated (en tomophilous) flowers, as in nearly all members of the Com positae (q.v.) and Umbelliferae, many Labiatae (such as dead nettle [Lamium] and Salvia), the Caryophyllaceae, the large willow-herb (Epilobium angustifolium), etc. Proterogyny is found in the horse chestnut (Aesculus), the autumn crocus (Col chicum), many Araceae, and in wind-pollinated anemophilous flow ers such as plaintain (Plantago), meadow rue (Thalictrum) and many grasses, though here separation in time is very short and many are self-pollinated as wheat, barley and oats. It is often accepted that cross-pollination is of greater value to the plant than self-pollination in respect of weight and number of seeds; the question is, however, one of some difficulty. The numerous pro visions in flowers for aiding cross-pollination and hindering self pollination suggest the superiority of the former process, but there are numerous plants which normally and for generations are self pollinated.