What do the pleated structures on a saguaro cactus do?

The pleated structures on a giant saguaro [Carnegiea gigantea] allow the plant to increase or decrease in size. This contraction or expansion is necessary in terms of the cactus' interior contents. The stem moves, processes and stores water, nutrient solutions, and energizing photosynthetic products. Greater amounts need more space, lesser amounts less. It's one more way in which the cactus adapts, survives, and succeeds in extreme, harsh desert environments.