There are three general classifications of growth habit present in grasses; bunch-type (also called caespitose), stoloniferous, and rhizomatous.
The success of the grasses lies in part in their morphology and growth processes, and in part in their physiological diversity. Most of the grasses divide into two physiological groups, using the C3 and C4 photosynthetic pathways for carbon fixation. The C4 grasses have a photosynthetic pathway linked to specialized Kranz leaf anatomy that particularly adapts them to hot climates and an atmosphere low in carbon dioxide.
C3 grasses are referred to as ‘cool season grasses’ while C4 plants are considered ‘warm season grasses’. Grasses may be either annual or perennial.
*Annual Cool Season - wheat, rye, Annual Bluegrass (annual meadowgrass, Poa annua), and oat
*Perennial Cool Season - orchardgrass (cocksfoot, Dactylis glomerata), fescue (Festuca spp), Kentucky Bluegrass and perennial ryegrass (Lolium perenne).
*Annual Warm Season - corn, sudangrass, and pearl millet.
*Perennial Warm Season - big bluestem, indiangrass, bermudagrass and switchgrass.
Until recently grasses were thought to have evolved around 55 million years ago, based on fossil records. However, recent findings of 65-million-year-old phytoliths resembling grass phytoliths (including ancestors of rice and bamboo) in Cretaceous dinosaur coprolites, may place the diversification of grasses to an earlier date.