Why do cacti have spines instead of normal leaves?

In dry environments, broad leaves would lose too much water through evaporation. Cacti evolved spines, which are modified leaves, to minimize surface area and reduce water loss. The spines also help deter herbivores and can cast shade or trap moisture near the plant surface. Because the leaves are reduced, the green stem does most of the photosynthesis and often stores water in succulent tissues. Cacti are especially diverse in the Americas, where they evolved to survive arid habitats.