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Yew pollen shower. Large quantity of pollen being released by male flowers of the common Yew (Taxus baccata) tree. Pollination by wind (amenophily) is common in plants such as this. For success, it requires vast quantities of pollen to be produced and the flowers in this picture will produce several million grains. Pollen from amenophilous plants is the principal cause of hay fever in humans. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR JEREMY BURGESS/SCIENCE PHOTO LIBRARY)