The transfer of pollen from anthers to stigma is called pollination. Cross-pollination is the transfer of pollen from the anthers of one flower to the stigma of another flower of the same species. In some species self-pollination occurs either regularly, as in the ground-nut, or when cross-pollination has failed to take place, as in Tridax. In cross-pollination pollen is usually transferred on the bodies of insects entering the fiowers, or by chance air currents carrying the pollen from one flower to the next. The structures of many flowers are closely adapted to the method of insect or wind pollination. Gives the main differences between these two kinds of flower.

Insect pollination mechanisms

   Essentially, these mechanisms involve an insect's visiting one flower, becoming dusted with pollen from the ripe stamens, and then visiting another flower where some of the pollen on its body adheres to the stigma. When ripe, the pollen sacs of the anther split open and expose the pollen which can then be dislodged.

    Self-pollination is prevented in various ways. In some flowers the anthers have shed all their pollen and have shrivelled before the stigma is receptive. In other cases the pollen of one flower Simply Will not germinate and produce a pollen tube on the stigma of its own flower. In certain species, the position of the stamens and style alter as the flower ages so that the visiting insect will come into contact with either the anthers, or the stigma but not both.

    Caesalpinia. This flower is probably pollinated by large moths and sunbirds. The sunbird's beak, in probing for nectar, becomes dusted with pollen from the anthers. When the bird Visits another flower, the pollen may adhere to the sticky surface of the stigma.

    Thunbergia. The colour and scent attract bees which enter the flower in search of nectar. As the bee crawls through the corolla tube the anthers brush the insect's back, dusting it with pollen. Similarly, on its way in, the bee encounters the stigma whose sticky surface picks up pollen which has been deposited on the insect from another flower.  Frequently a bee, e.g. a carpenter bee, gains access to the nectary by biting through the corolla tube from the outside and inserting its proboscis ("tongue). This robs the plant of its nectar without bringing about pollination.

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    Crotalaria and other bean family flowers. Some members of this family have no nectar, and the bees or moths which visit them collect only pollen from the flowers. Other members including Crotalaria do produce nectar.

   The weight of the insect depresses the wings" of the flower as it alights on them. Near their bases, the wings are linked to the petals of the keel so that these too are forced down. Pollen from the long anthers has collected at the tip of the keel and as the latter is forced down, the accumulated pollen is forced out of a hole in its tip by the rounded anthers and hairy stigma. Thus a ribbon of pollen is extruded on the under-surface of the bee. In the nectar-producing members of this family, the sheath of filaments round the ovary is open at the top, enabling the insect'stongue" to reach the nectaries.

    When an insect visits a flower from whose keel all pollen has been expelled, only the stigma will emerge, touch the underside of the insect and collect any polen that has adhered froma previous Visits to Crotalaria flowers. Clearly, a great deal of pollen from the same flower will come into contact with the stigma, and in Crotalaria probably results in self-pollination. In many other species, however, chemicals in the stigma prevent the growth of the pollen; the pollen and stigma are self- incompatible.

    Zea mays. When mature, the filaments in the male flowers elongate so that the anthers hang outside the glumes. The pollen sacs split open and pollen is carried away in air currents, some of it becoming trapped on the feathery styles of the female flowers which present a large surface area. Since in any one maize plant the anthers are ripe before the styles are receptive, pollination is unlikely to occur between the flowers of the same plant, while cross- pollination between neighbouring plants is favoured.


In both wind- and insect-pollinated flowers, pollen from a certain species may reach the stigma of a different species. Usually the chemicals present in the cells of the stigma prevent further development of the "foreign" pollen grains.

Importance of pollination to agriculture

  After fertilization the ovary of a plant develops into a fruit. Fruit formation therefore depends on fertilization, which can follow only after pollination.

    Farmers and fruit growers are well aware that a good yield of fruit will occur only if most of the available flowers have been pollinated. Many of the cereals are self-pollinated or wind-pollinated, and the fact that the plants grow close together makes the latter effective.


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