Stomata of CAM plants:

Stomata usually open when leaves are transferred from darkness to light, i.e. at night. Plants such as cacti and Opuntia achieve their high water use efficiency by opening their stomata during the cool, desert nights and closing them during the hot, dry days. Related Theory In CAM plants or xerophytes the stomata open in night and remain closed during day as an adaptation to conserve water. The opening of stomata in these plants occurs at night to collect carbon dioxide and allow it to diffuse to the mesophyll cells. Carbon dioxide is stored in the form of malic acid in vacuoles at night.