Why do cacti and desert plants have spines, and not leaves?

Most cacti do not have leaves because they grow in a hot, dry environment, with little water. They are able to survive by storing water for long periods of time; they do this through their extensive root system, which absorbs water from the soil when it rains. In a typical leaf, there is a large amount of water loss through small openings at the leaf surface (called the stomatas). This water loss, called transpiration, is speeded up with warm temperatures. Cacti have adapted by being leafless, storing water in their stems, developing waxy skin to seal in moisture, and growing spines, a form of leaf modification. Spines help shade the rounded or ribbed stems from the blazing sun of their desert habitat. They also protect desert animal species by providing them with shelter.