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Euphorbia (Euphorbia spiralis). This succulent plant is endemic to the Socotran archipelago of islands, meaning that it is found nowhere else. Succulent plants are adapted to arid conditions by having swollen, water-storing stems and leaves. Socotra is located in the Indian Ocean at the mouth of the Gulf of Aden, 340 kilometres (km) from mainland Yemen and 250km from the coast of Somalia. Its isolation has led to the evolution of a unique flora and fauna. (KEYSTONE/SCIENCE PHOTO LIBRARY/DICCON ALEXANDER)