JOHN MEAD / SCIENCE PHOTO LIBRARY JOHN MEAD / SCIENCE PHOTO LIBRARY
A sahuaro cactus, Cereus giganteus, growing in the desert at Tucson, Arizona, USA. This species is the best known and the largest of all the cacti, some specimens reaching 15 metres in height. The fleshy body typical of the cactus family, Cactaceae, is in fact an enlarged stem. This is capable of storing large quantities of water, enabling the plant to survive in arid regions. Due to the mucilaginous nature of the cell contents and to the low surface area of these plants, the stored water is lost very slowly.
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