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Shower of pollen. Wind dispersal of pollen from male catkin flowers of an oak tree, Quercus sp. A shower of pollen falls in a light wind. Catkins are pendulous flowers of some trees, which are specially adapted to maximize pollen dispersal by wind. Oak trees flower in May-June producing copious quantities of pollen which is a major cause of hay fever (medical term Allergic rhinitis and pollenosis). (KEYSTONE/SCIENCE PHOTO LIBRARY/DR JEREMY BURGESS)