Pollination is thus the transfer of pollen from male structures to female structures of the same species. Pollination is accomplished by several methods, the commonest method in flowering plants is by insects. In these cases, flowers may be showy, colourful, fragrant and otherwise attractive to the pollinating agents. The pollen may be large, sculptured and often with an adhesive coating. The pollen which are important in aerobiology are from plants in which wind is the pollinating agent. In these flowering plants, the flowers are usually small, inconspicuous, numerous and without odour. Such pollen grains are mostly small, smooth and non-adhesive. Airborne pollen is usually produced in large quantities. The flowers or clusters of flowers may be on long stalks that move with the wind. The filaments of the anthers may be long and flexuous.
Some airborne pollen are slightly adhesive and may be carried by both wind and insects as in Tilia (basswood or linden), Acer (maple), Salix (willow) and Castanea (chestnut), Parthenium (Congress grass). The pollen may stick together and are found in clumps on the samples e.g. Ambrosia, Peltophorum, Parthenium. It is mainly the windborne pollen grains that have a bearing in pollinosis, because they more easily come in contact with the hypersensitive tissues of human beings than other types of pollen.
Insect-borne pollen grains in general, are not produced in such abundance as the windborne ones, and for that reason, they are of lesser importance as causes of pollinosis. This does not mean however, that they can be completely ignored. It has been observed in a few instances that pollen from entomophilous plants like Carica papaya and Argemone mexicana, produced symptoms of pollinosis in patients (Shivpuri and Dua 1963, 1964; Agashe 1989).
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