Deserts are the ultimate step in loss of vegetation, due to climatic or human conditions that almost prevent the growth of plants. There are different degrees of desert, and the usage of the term has many local variants. What one person in the USA might refer to as "desert" would be considered much too densely vegetated to be called desert in north Africa, where ecologists have the extremely arid Sahara as their standard. Many ecologists from around the world would say that there is no "real" desert anywhere in the USA, because even the driest areas have too much vegetation!
Deserts often have plants which have done away with leaves and instead have swollen stems that store water in large bag-like cells. Or, if they do have leaves, they are swollen and distended, also full of water. This is the "succulent" growth habit, which functions as a reservoir. Water is taken up by the plant's roots when its rains, and this store in its stems and leaves keeps it going for months or years until the next rain storm. Most people call any succulent plant like this a "cactus", but in fact cacti only occur naturally in deserts in the Americas. Several other groups of plants, some of them loosely related to the "true" cacti of the Americas, occur in dry climates in Africa. One example is the curious "living stones'' genus Lithops in the Namib Desert of southwest Africa, which disguise themselves as pebbles to avoid being eaten by herbivores.
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