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Wood anemone flower (Anemone nemorosa). This small flower averages around 2 centimetres in diameter. It starts out a pink colour, and then turns white and exposes its pollen-bearing anthers (yellow). The pollen contains the male gametes (sex cells), and is formed as part of the plant's reproductive cycle. Wood anemone flowers are a good indicator of ancient, undisturbed woodland habitat. This one was photographed in Hareshaw Dene SSSI, a Site of Special Scientific Interest, in Northumberland National Park, England. SSSIs are selected for the diversity of their plants and animals. (KEYSTONE/SCIENCE PHOTO LIBRARY/SIMON FRASER)