The CAM lineages of planet Earth.

We found direct evidence - in the form of experimental or field observations of gas exchange, day-night fluctuations in organic acids, carbon isotope ratios, and enzymatic activity - for CAM in 370 genera of vascular plants, representing 38 families. Further assumptions about the frequency of CAM species in CAM clades, and the distribution of CAM in the Cactaceae and Crassulaceae, bring the currently estimated number of CAM-capable species to nearly 7% of all vascular plants. The phylogenetic distribution of these taxa suggests a minimum of 66 independent origins of CAM in vascular plants, with possibly dozens more. To achieve further insight into CAM origins, there is a need for more extensive and systematic surveys of previously unstudied lineages, particularly in living material to identify low-level CAM activity, and for denser sampling to increase phylogenetic resolution in CAM-evolving clades. This should allow further progress in understanding the functional significance of this pathway by integration with studies on the evolution and genomics of CAM in its many forms.