Under natural conditions, the acid content of certain succulent plants increases at night and decreases the following day. This type of diurnal variation in acidity was first discovered in the 1950s in Crassula species, hence the name crassulacean acid metabolism (CAM). This important strategy enables some plants, such as cacti, bromeliads, euphorbias, and thick-leaved orchids to minimize water loss. Well over 5% of vascular plant species engage in this photosynthetic pathway. Ecologically, they are best represented in exposed terrestrial and epiphytic habitats where periodic drought stress is combined with high solar radiation.

CAM permits plants to control water loss by restricting the exchange of carbon dioxide, oxygen, and water vapor to cooler, evening hours. CAM plants photosynthesize during the day, when their stomatal pores are closed. To accomplish this, carbon dioxide is absorbed at night, then fixed as malic acid. The acid is stored as liquid in sacs (vacuoles) within special cells in the leaf, stem, or pseudobulb. The next day, the acid is metabolized, and photosynthesis takes place.

In CAM, much of the CO₂ for photosynthesis is taken up at night via the C₄ carboxylating enzyme PEP carboxylase (PEPC). During the day the decarboxylation of organic acids releases CO₂, which is refixed by the C₃ carboxylating enzyme RubisCO when stomata are usually closed, thereby conserving water. The CO₂ cycled through the CAM pathway may also have an important photoprotective role, particularly in the absence of net daytime photosynthesis.

Sources: http://www.prysm.net/~orchidsafari/archiv14.html#1; http://www.ncl.ac.uk/aes/html/cam_metabolism.html; see also http://www.biols.susx.ac.uk/Home/Peter_Scott/LECT_09/LECT_09.HTM