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Joshua tree (Yucca brevifolia) fruits. Joshua trees bloom in early spring, when clusters of flowers develop at the ends of the branches. Once pollinated, each flower develops into a fleshy fruit containing seeds from which new plants will grow. These yucca plants live only in south- western USA and north Mexico. They are most common in the Mojave desert of California, at altitudes over 900 metres. (KEYSTONE/SCIENCE PHOTO LIBRARY/DICCON ALEXANDER)