Cacti include over 200 species spread over the hot and dry regions of North and South America, east and West Africa and Asia. Desserts are called arid zones. The steam is green and usually bears no leaf which in many species turns into brownish or grayish spines which grow to great lengths. The spines make extremely formidable weapons of defense against animals that might otherwise be tempted to feed on the green stem. The spines are borne on enlarged, cushion – like areoles, some species also bear very fine, barbed bristles. The function of these modified leaves is to protect the green stem from injury. It is an important task since stems assume the charge of producing the organic food which is normally done by the leaves. The thick, flashy, leafless and green stem also stores precious water for running the life activities of the planet. A waxy layer on stem prevents evaporation of water though transpiration. In the absence of spines on cacti, other protective adaptations are present. The outer covering of the plant is exceedingly tough. There may be poisonous substance and the plants may be protectively colored. The stem may be flattened or they may be ridged and grooved. The common kinds of cacti include prickly pears, the giant cactus or saguaro and cholla types.