Poaceae

Growth and development

Grass blades grow at the base of the blade and not from elongated stem tips. This low growth point evolved in response to grazing animals and allows grasses to be grazed or mown regularly without severe damage to the plant.^{[11]: 113–114}

Three general classifications of growth habit present in grasses: bunch-type (also called caespitose), stoloniferous, and rhizomatous.^[12] The success of the grasses lies in part in their morphology and growth processes and in part in their physiological diversity. There are both C3 and C4 grasses, referring to the photosynthetic pathway for carbon fixation. The C4 grasses have a photosynthetic pathway, linked to specialized Kranz leaf anatomy, which allows for increased water use efficiency, rendering them better adapted to hot, arid environments.^[13]

The C3 grasses are referred to as "cool-season" grasses, while the C4 plants are considered "warm-season" grasses.^{[8]: 18–19}

• 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 – maize, sudangrass, and pearl millet
• Perennial warm-season – big bluestem, Indiangrass, Bermudagrass and switchgrass.

Although the C4 species are all in the PACMAD clade (see diagram below), it seems that various forms of C4 have arisen some twenty or more times, in various subfamilies or genera. In the Aristida genus for example, one species (A. longifolia) is C3 but the approximately 300 other species are C4. As another example, the whole tribe of Andropogoneae, which includes maize, sorghum, sugar cane, "Job's tears", and bluestem grasses, is C4.^[14] Around 46 percent of grass species are C4 plants.^[15]

Evolutionary history

Grasses include some of the most versatile plant life-forms. They became widespread toward the end of the Cretaceous period, and fossilized dinosaur dung (coprolites) have been found containing phytoliths of a variety that include grasses that are related to modern rice and bamboo.^[17] Grasses have adapted to conditions in lush rain forests, dry deserts, cold mountains and even intertidal habitats, and are currently the most widespread plant type; grass is a valuable source of food and energy for all sorts of wildlife.

A cladogram shows subfamilies and approximate species numbers in brackets:^[14]

Before 2005, fossil findings indicated that grasses evolved around 55 million years ago. Finds of grass-like phytoliths in Cretaceous dinosaur coprolites from the latest Cretaceous (Maastrichtian) aged Lameta Formation of India have pushed this date back to 66 million years ago.^[18][19] In 2011, fossils from the same deposit were found to belong to the modern rice tribe Oryzeae, suggesting substantial diversification of major lineages by this time.^[20]

In 2018, a study described grass microfossils extracted from the teeth of the hadrosauroid dinosaur Equijubus normani from northern China, dating to the Albian stage of the Early Cretaceous approximately 113–100 million years ago, which were found to belong to primitive lineages within Poaceae, similar in position to the Anomochlooideae. These are currently the oldest known grass fossils.^[1]

Fossils of Phragmites have been found in the Late Cretaceous of North America, particularly in the Maastrichtian aged Laramie Formation.^[21] However slightly older fossils of Phragmites have been found in the Eastern coast of the US dating the Campanian (such as in the Black Creek Formation). ^[22]

The relationships among the three subfamilies Bambusoideae, Oryzoideae and Pooideae in the BOP clade have been resolved: Bambusoideae and Pooideae are more closely related to each other than to Oryzoideae.^[23] This separation occurred within the relatively short time span of about 4 million years.

According to Lester Charles King, the spread of grasses in the Late Cenozoic would have changed patterns of hillslope evolution favouring slopes that are convex upslope and concave downslope and lacking a free face were common. King argued that this was the result of more slowly acting surface wash caused by carpets of grass which in turn would have resulted in relatively more soil creep.^[24][25]

Subdivisions

There are about 12,000 grass species in about 771 genera that are classified into 12 subfamilies.^[26] See the full list of Poaceae genera.

• Anomochlooideae Pilg. ex Potztal, a small lineage of broad-leaved grasses that includes two genera (Anomochloa, Streptochaeta)
• Pharoideae L.G.Clark & Judz., a small lineage of grasses of three genera, including Pharus and Leptaspis
• Puelioideae L.G.Clark, M.Kobay., S.Mathews, Spangler & E.A.Kellogg, a small lineage of the African genus Puelia
• Pooideae, including wheat, barley, oats, brome-grass (Bromus), reed-grasses (Calamagrostis) and many lawn and pasture grasses such as bluegrass (Poa)

• Bambusoideae, including bamboo
• Ehrhartoideae, including rice and wild rice
• Aristidoideae, including Aristida
• Arundinoideae, including giant reed and common reed
• Chloridoideae, including the lovegrasses (Eragrostis, about 350 species, including teff), dropseeds (Sporobolus, some 160 species), finger millet (Eleusine coracana (L.) Gaertn.), and the muhly grasses (Muhlenbergia, about 175 species)
• Panicoideae, including panic grass, maize, sorghum, sugarcane, most millets, fonio, "Job's tears", and bluestem grasses
• Micrairoideae
• Danthonioideae, including pampas grass

Food production

Of all crops grown, 70% are grasses.^[38] Agricultural grasses grown for their edible seeds are called cereals or grains (although the latter term, when used agriculturally, refers to both cereals and similar seeds of other plant species, such as buckwheat and legumes). Three cereals—rice, wheat, and maize (corn)—provide more than half of all calories consumed by humans.^[39] Cereals constitute the major source of carbohydrates for humans and perhaps the major source of protein; these include rice (in southern and eastern Asia), maize (in Central and South America), and wheat and barley (in Europe, northern Asia and the Americas).

Sugarcane is the major source of sugar production. Additional food uses of sugarcane include sprouted grain, shoots, and rhizomes, and in drink they include sugarcane juice and plant milk, as well as rum, beer, whisky, and vodka.

Bamboo shoots are used in numerous Asian dishes and broths, and are available in supermarkets in various sliced forms, in both fresh, fermented and canned versions.

Lemongrass is a grass used as a culinary herb for its citrus-like flavor and scent.

Many species of grass are grown as pasture for foraging or as fodder for prescribed livestock feeds, particularly in the case of cattle, horses, and sheep. Such grasses may be cut and stored for later feeding, especially for the winter, in the form of bales of hay or straw, or in silos as silage. Straw (and sometimes hay) may also be used as bedding for animals.

An example of a sod-forming perennial grass used in agriculture is Thinopyrum intermedium.

Industry

Grasses are used as raw material for a multitude of purposes, including construction and in the composition of building materials such as cob, for insulation, in the manufacture of paper and board such as oriented structural straw board. Grass fiber can be used for making paper, biofuel production,^[40] nonwoven fabrics, and as replacement for glass fibers used in reinforced plastics.^[41] Bamboo scaffolding is able to withstand typhoon-force winds that would break steel scaffolding.^[29] Larger bamboos and Arundo donax have stout culms that can be used in a manner similar to timber, Arundo is used to make reeds for woodwind instruments, and bamboo is used for innumerable implements.^[42]

Phragmites australis (common reed) is important for thatching and wall construction of homes in Africa.^[43] Grasses are used in water treatment systems,^[44] in wetland conservation and land reclamation, and used to lessen the erosional impact of urban storm water runoff.^[45]

Palaeoecological reconstructions

Pollen morphology, particularly in the Poaceae family, is key to figuring out their evolutionary relationships and how environments have changed over time.^[46] Grass pollen grains, however, often look the same, making it hard to use them for detailed climate or environmental reconstructions.^[47][48] Grass pollen has a single pore and can vary a lot in size, from about 20 to over 100 micrometers, and this size difference has been looked into for clues about past habitats, to tell apart domesticated grasses from wild ones,^[49][50][51] and to indicate various biological features like how they perform photosynthesis,^[52] their breeding systems,^[52][53] and genetic complexity.^[54][55][47] Yet, there's ongoing debate about how effective pollen size is for piecing together historical landscapes and weather patterns, considering other factors such as genetic material amount might also affect pollen size.^[56][57][47] Despite these challenges, new techniques in Fourier-Transform Infrared Spectroscopy (FT-IR) and improved statistical methods are now helping to better identify these similar-looking pollen types.^[48]

Lawn and ornamental use

Grasses are the primary plants used in lawns, which themselves derive from grazed grasslands in Europe.^{[citation needed]} They also provide an important means of erosion control (e.g., along roadsides), especially on sloping land.^{[citation needed]} Grass lawns are an important covering of playing surfaces in many sports, including football (soccer), American football, tennis, golf, cricket, softball and baseball.

Ornamental grasses, such as perennial bunch grasses, are used in many styles of garden design for their foliage, inflorescences and seed heads. They are often used in natural landscaping, xeriscaping and slope and beach stabilization in contemporary landscaping, wildlife gardening, and native plant gardening.^{[citation needed]} They are used as screens and hedges.^[58]

Sports turf

Grass playing fields, courses and pitches are the traditional playing surfaces for many sports, including American football, association football, baseball, cricket, golf, and rugby. Grass surfaces are also sometimes used for horse racing and tennis. Type of maintenance and species of grass used may be important factors for some sports, less critical for others. In some sports facilities, including indoor domes and other places where maintenance of a grass field would be difficult, grass may be replaced with artificial turf, a synthetic grass-like substitute.^[59]

Cricket

Main article: Cricket pitch § Preparation and maintenance of the playing area

In cricket, the pitch is the strip of carefully mowed and rolled grass where the bowler bowls. In the days leading up to the match it is repeatedly mowed and rolled to produce a very hard, flat surface for the ball to bounce off.^[60]

Economically important grasses

A number of grasses are invasive species that damage natural ecosystems, including forms of Phragmites australis which are native to Eurasia but has spread around the world.^[62][63]