why does cactus have so many Thorns

The thorns or spines of cactus is actually the leaf of the cactus plant.Cactus has undergone adaptation. Usually cactus grown in dry area and there will be more evaporation takes place from leaves .So when leaf is present the water from the cactus plant totally will be get evaporised.Hence to avoid this cactus modified its leaf to thorns. Photosynthesis in cactus take place in the stem of the cactus plant.