What is on the outside of a cactus stem?

The outside of the cactus stem is covered with a thickened outer skin that may be glossy, leathery or waxy. The skin protects the cactus from excess water loss through evaporation. It also serves to deflect and disperse light, through its ribbing. The ribbing also allows the stem to contract or expand to accommodate more or less amounts of water. The skin is a source for growth, through its growth buds. From the buds grow the branches, modified leaves, fruits, flowers, and seeds.