what are thorns on cacti composed of,so that transpiration is decreased?

Cacti leaves typically are reduced in the size so that they reduce the surface area through which water can be lost by transpiration. In some species, the leaves are still remarkably large and ordinary while in other species they have became microscopic but they still contain the stomata, xylem, and phloem. Cactus have also developed spines, which allow less water to evaporate through transpiration by shading the plant, and defend the cactus against water-seeking animals. The spines grow from specialized structures called areoles.