Water collected during the wet period can be amassed in large amounts and stored to be used over the entire dry season. Food manufacturing has been taken over by the stem. The stems are green and photosynthetic and replace the function of the leaves. Cacti stems have many stomata that are closed in the day and open at night. This limits transpiration and water loss during the day when temperatures are at their maximum and relative humidity is low.
Evaporation is much less likely at night.
The stem is composed of lots of cortex and pith tissue, which provide a reservoir for food and water. There is only a very little amount of woody tissue that acts as a support of the swollen stem, the rest of the tissue is used for storage. This stored water is used sparingly during drought, most of the year. Astrophytum sp., have pleated stems that swell when they take in a lot of water, and shrink during the dry months as they use up their supply. The epidermal layer of the cacti stem is tough and can allow for this shrink and swell.