Cacti occur in a wide range of shapes and sizes as do their flowers. Many live in extremely dry environments, even being found in the Atacama Desert, one of the driest places on earth. Almost all cacti are succulents, but unlike many other succulents, the stem is the only part of most cacti where photosynthesis takes place. Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves. As well as defending against herbivores, spines help prevent water loss by reducing air flow close to the cactus and providing some shade. In the absence of leaves. Cactus flowers are pollinated by insects, birds and bats. None are known to be wind-pollinated and self-pollination occurs in only a very few species. Favorite edible cacti are prickly pear and dragon fruit. A number of species of cacti have been shown to contain psychoactive agents. Two species have a long history of use by the indigenous peoples of the Americas are peyote, Lophophora williamsii, in North America, and the San Pedro cactus, Echinopsis pachanoi, in South America. Both contain mescaline.