Don't be frightened by natural "corking". One aspect of the cactus life cycle with which most people are not familiar is the process of "corking", in which the bottom portions of a mature cactus slowly start to develop a tough, brown, bark-like exterior. Though this condition can appear serious because it replaces the natural green exterior with one that appears dead, it's not actually a sign that the plant is in any danger and can usually be ignored.