 By studying the ratio of 14C to 13C incorporated into a plant — its isotopic signature — it is possible to deduce how much CO2 is taken up at night and how much in the daytime. Using this approach, most of the Pereskia species investigated exhibit some degree of CAM-cycling, suggesting this ability was present in the ancestor of all cacti. Pereskia leaves are claimed to only have the C3 mechanism with CAM restricted to stems. More recent studies show that "it is highly unlikely that significant carbon assimilation occurs in the stem"; Pereskia species are described as having "C3 with inducible CAM." Leafless cacti carry out all their photosynthesis in the stem, using full CAM. As of February 2012, it is not clear whether stem-based CAM evolved once only in the core cacti, or separately in the opuntias and cactoids; CAM is known to have evolved convergently many times.
