C3 plants are those which fix and reduce inorganic CO2 into organic compounds using only the C3 pathway in photosynthesis while C4 and CAM plants employ both C3 and C4 cycles. In other words, the first classification refers to those plants having C3 photosynthesis, C4 plants employ the C4 photosynthesis, and CAM plants the CAM photosynthesis.

Plants utilizing only the C3 cycle are most common in the Plant kingdom. They comprise about 85% of all plant species (Moore et al. 2003). In contrast, only about 3% are C4 plants while about 8% were identified as CAM plants as of 2010 (Simpson 2010).

Table C-1. Some characteristics and the general distribution of the three plant types grouped according to their mechanisms of photosynthesis in the Dark reactions.
Distribution in the plant kingdom (% of plant species) ~85% (Moore et al. 2003) ~3% (Simpson 2010), all angiospermous including most troublesome weeds; mostly monocots (C4 grasses and sedges about 79% of all C4 plants) ~8% (Simpson 2010), mostly succulent plants but not all succulents are CAM plants
Type of photosynthesis C3 photosynthesis C4 photosynthesis CAM photosynthesis

CO2 fixation pathway via C3 cycle only via C3 and C4 cycles, spatially (C4 in the mesophyll cell then C3 in the bundle sheath cell) via C3 and C4 cycles, both spatially (in different parts of same cell) and temporally (C4 at night, C3 at day time)
Leaf anatomy Large air spaces bordered by loosely arranged spongy mesophyll cells; mesophyll cells but not bundle sheath cells (BSC) contain chloroplasts Generally thinner leaves, closer arrangement of vascular bundles, smaller air spaces than C3; veins sorrounded by thick-walled BSC further sorrounded by thin-walled mesophyll cells (wreath-like arrangement of BSC is called Kranz anatomy); mesophyll cells and BSC contain chloroplasts, those of the BSC much larger Thick and fleshy leaves, mesophyll cells having large, water-filled vacuoles
Stomatal movement Stomata open at daytime, close at night Stomata open at daytime, close at night Inverted stomatal cycle (open at night, close in the day)
Typical Environmental / Geographical adaptation (where most common) Temperate Tropical or semi-tropical, high light intensity, high temperature, drought conditions Desert or arid (xeric) habitats

Examples of C3 plants:

- most small seeded cereal crops such as rice (Oryza sativa), wheat (Triticum spp.), barley (Hordeum vulgare), rye (Secale cereale), and oat (Avena sativa); soybean (Gycine max), peanut (Arachis hypogaea), cotton (Gossypium spp.), sugar beets (Beta vulgaris), tobacco (Nicotiana tabacum), spinach (Spinacea oleracea), potato (Solanum tuberosum); most trees and lawn grasses such as rye, fescue, and Kentucky bluegrass.

Also includes evergreen trees and shrubs of the tropics, subtropics, and the Mediterranean; temperate evergreen conifers like the Scotch pine (Pinus sylvestris); deciduous trees and shrubs of the temperate regions, e.g. European beech (Fagus sylvatica) (Moore et al. 2003), as well as weedy plants like the water hyacinth (Eichornia crassipes), lambsquarters (Chenopodium album), bindweed (Convolvolus arvensis), and wild oat (Avena fatua) (Llewellyn 2000).

(Ben G. Bareja Aug 2013)