spines help prevent water loss by reducing air flow close to the cactus and providing some shade.Spines help the cactus in several ways. The most obvious use of the
spines is for protection against foragers. The cactus basically states
"How much pain are you willing to indure to get a drink in this
desert?"; Surprisingly, some animals do partake. In some cactus
varieties the spines participate in water collection. Water from dew
condenses on spines and, in some cactus species, downward-pointing
spines help to direct rainwater to the roots of the plant. Spines help
to reflect light away from the cactus stem theoretically lowering the
stem temperature. Some spines also trap in a layer of air next to the
cactus stem preventing loss of water via evaporative cooling.