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Arctic willow flower (Salix arctica), close-up. This shrub-like perennial grows in the Arctic tundra. Each plant produces only all male or all female flowers, known as catkins. Each catkin is composed of tiny flowers that lack petals. Male catkins produce pollen whilst female catkins contain ovules. Male sex cells in the pollen fertilise the ovules, producing hairy seeds that are dispersed by the wind. Photographed on Clavering Island in North East Greenland National Park, Greenland. (KEYSTONE/SCIENCE PHOTO LIBRARY/SIMON FRASER)