The answer is 'spines to reduce water loss'. Spines are in effect a specialised form of leaf. Obviously, spines have a much reduced surface area compared to a leaf, thereby drastically reducing evapotranspiration. The spines also have an important protective function for the plant. Other adaptions by desert plants would include retaining water (such as succulents), a waxy cuticle on leaves to reduce water loss and closure of the stomata to reduce water loss.
