text: The C metabolic pathway is a valuable recent evolutionary innovation in plants, involving a complex set of adaptive changes to physiology and gene expression patterns. About 7600 species of plants use  carbon fixation, which represents about 3% of all terrestrial species of plants.  All these 7600 species are angiosperms.
  C plants evolved carbon concentrating mechanisms. These work by increasing the concentration of  around RuBisCO, thereby facilitating photosynthesis and decreasing photorespiration. The process of concentrating  around RuBisCO requires more energy than allowing gases to diffuse, but under certain conditions – i.e. warm temperatures (>25 °C), low  concentrations, or high oxygen concentrations – pays off in terms of the decreased loss of sugars through photorespiration.
  One type of C metabolism employs a so-called Kranz anatomy. This transports  through an outer mesophyll layer, via a range of organic molecules, to the central bundle sheath cells, where the  is released. In this way,  is concentrated near the site of RuBisCO operation. Because RuBisCO is operating in an environment with much more  than it otherwise would be, it performs more efficiently. In C photosynthesis, carbon is fixed by an enzyme called PEP carboxylase, which, like all enzymes involved in C photosynthesis, originated from non-photosynthetic ancestral enzymes.
  A second mechanism, CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. The most important benefit of CAM to the plant is the ability to leave most leaf stomata closed during the day. This reduces water loss due to evapotranspiration. The stomata open at night to collect , which is stored as the four-carbon acid malate, and then used during photosynthesis during the day. The pre-collected  is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency. More  is then harvested from the atmosphere when stomata open, during the cool, moist nights, reducing water loss.
  CAM has evolved convergently many times. It occurs in 16,000 species (about 7% of plants), belonging to over 300 genera and around 40 families, but this is thought to be a considerable underestimate. It is found in quillworts (relatives of club mosses), in ferns, and in gymnosperms, but the great majority of plants using CAM are angiosperms (flowering plants).