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Pollen production. Cloud of pollen (white) being produce by the shaking of male flowers (catkins) of the hazel plant (Corylus avellana). Wind pollination is known as amenophily. For success, it requires vast quantities of pollen to be produced. A single hazel catkin is estimated to produce up to 4 million pollen grains. Successful pollination leads to the formation of a seed that may form a new plant. Wind-borne pollen is the principal cause of hay fever in humans. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR JEREMY BURGESS/SCIENCE PHOTO LIBRARY)