Why do cacti have needles?

Answer #1 The cactus has needles to lessen the loss of moisture through evaporation. Needles are the cactus plant's equivalent of leaves. The smaller the leaf or needle surface, the less the loss of precious moisture through evaporation. The needles also let moisture and rain water drip to the soil surface. The water then seeps below ground. Underground, the water adds to soil moisture levels and is taken in by plant roots. It dissolves nutrients that only can be used in soluble form by plants. Then it travels up through the plant by capillary action, for the meeting with sunlight in the photosynthetic interaction. Additionally, the needles offer general protection to cactus body parts from such environmental stressors as pests. Specifically, it may protect fruits, flowers, and fleshy parts from predators. Answer #2Cactuses are prickly so the animals stay out of the way, and so the animals can't drink their water up.