Scanning electron micrograph of a catkin of the hazel, Corylus avellana. The picture is a close view (6x4mm) of a catkin, that consists of a drooping stem bearing a series of male flowers. Each flower has a hairy scale (brown) and below bears anthers (green) containing pollen grains (magenta). Hazel is monoecious: it also bears female flowers. These appear as small buds with protruding red stigmas, not shown here. The female flowers are wind-pollinated. It is an inefficient process, necessitating the production of large amounts of pollen. A single hazel catkin releases about 4 million pollen grains, very few of which will cross-pollinate a female flower on a different hazel bush. Self pollination is minimised by weak self incompatibility that inhibits pollen tube growth, and by protogyny - female flowers mature before most catkins on the same bush are ready to release their pollen.