Why would a cactus shrink in the middle?

Answer 1

When water is available, cacti absorb more than they need for the present use. Their tissues, particularly in the thick body or trunk of the cacti, and in the thick pads of the Prickly Pear, swell up like sponges. When the weather dries and no water is available through their roots, cacti use this stored water. Their tissues then shrink up as the moisture is used, and the stem and/or pads become noticably smaller and wrinkled.

Answer 2

Having to use its stored water reserves may cause a cactus plant to shrink in the middle.

Specifically, there are a number of reasons for why a cactus plant may draw on its stored water reserves. For example, the plant may be able to capture a lot of rainfall and soil moisture. The stem is built precisely to expand to make room for greater water reserves. But that stem will contract, particularly in the middle, when those reserves are being used up.

A damaging cut may remove cactus parts devoted to growth. Growth cannot take place without at least one areole and one meristem. The absence of these parts can send confusing messages to the plant in terms of where stored water is needed and to which part it must be moved. To prevent such a situation, it is important to keep in mind that the best cut is always on the diagonal.

Or a cactus may shrink in response to damage (other than by human decision) to the roots or to the top. Such damage may keep dissolved nutrients and water from getting to the stem where energizers are made from the photosynthetic interaction with sunlight. The stress of available sunlight not having dissolved nutrients with which to interact may show up by shrinkage in the middle of the cactus stem.