Poaceae (/poʊˈeɪsiaɪ/) or Gramineae is a large and nearly ubiquitous family of monocotyledonous flowering plants known as grasses, commonly referred to collectively as grass. Poaceae includes the cereal grasses, bamboos and the grasses of natural grassland and cultivated lawns and pasture. Grasses have stems that are hollow except at the nodes and narrow alternate leaves borne in two ranks. The lower part of each leaf encloses the stem, forming a leaf-sheath. With around 780 genera and around 12,000 species, Poaceae are the fifth-largest plant family, following the Asteraceae, Orchidaceae, Fabaceae and Rubiaceae.
Grasslands such as savannah and prairie where grasses are dominant are estimated to constitute 40.5% of the land area of the Earth, excluding Greenland and Antarctica. Grasses are also an important part of the vegetation in many other habitats, including wetlands, forests and tundra. The Poaceae are the most economically important plant family, providing staple foods from domesticated cereal crops such as maize, wheat, rice, barley, and millet as well as forage, building materials (bamboo, thatch, straw) and fuel (ethanol).
Though they are commonly called "grasses", seagrasses, rushes, and sedges fall outside this family. The rushes and sedges are related to the Poaceae, being members of the order Poales, but the seagrasses are members of order Alismatales.
|Flowering head of meadow foxtail (Alopecurus pratensis), with stamens exerted at anthesis|
The name Poaceae was given by John Hendley Barnhart in 1895,:7 based on the tribe Poeae described in 1814 by Robert Brown, and the type genus Poa described in 1753 by Carl Linnaeus. The term is derived from the Ancient Greek πόα (póa, "fodder").
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. 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 and organics.
A cladogram shows subfamilies and approximate species numbers in brackets:
Before 2005, fossil findings indicated that grasses evolved around 55 million years ago. Recent findings of grass-like phytoliths in Cretaceous dinosaur coprolites have pushed this date back to 66 million years ago. In 2011, revised dating of the origins of the rice tribe Oryzeae suggested a date as early as 107 to 129 Mya.
Wu, You & Li (2018) described grass microfossils extracted from a specimen of the hadrosauroid dinosaur Equijubus normani from the Early Cretaceous (Albian) Zhonggou Formation (China). The authors noted that India became separated from Antarctica, and therefore also all other continents, approximately at the beginning of late Aptian, so the presence of grasses in both India and China during the Cretaceous indicates that the ancestor of Indian grasses must have existed before late Aptian. Wu, You & Li considered the Barremian origin for grasses to be probable
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. 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.
Grasses may be annual or perennial herbs,:10 generally with the following characteristics (the image gallery can be used for reference): The stems of grasses, called culms, are usually cylindrical (more rarely flattened, but not 3-angled) and are hollow, plugged at the nodes, where the leaves are attached. Grass leaves are nearly always alternate and distichous (in one plane), and have parallel veins.:11 Each leaf is differentiated into a lower sheath hugging the stem and a blade with entire (i.e., smooth) margins.:11 The leaf blades of many grasses are hardened with silica phytoliths, which discourage grazing animals; some, such as sword grass, are sharp enough to cut human skin. A membranous appendage or fringe of hairs called the ligule lies at the junction between sheath and blade, preventing water or insects from penetrating into the sheath.:11
Flowers of Poaceae are characteristically arranged in spikelets, each having one or more florets.:12 The spikelets are further grouped into panicles or spikes. The part of the spikelet that bears the florets is called the rachilla. A spikelet consists of two (or sometimes fewer) bracts at the base, called glumes, followed by one or more florets.:13 A floret consists of the flower surrounded by two bracts, one external—the lemma—and one internal—the palea. The flowers are usually hermaphroditic—maize being an important exception—and mainly anemophilous or wind-pollinated, although insects occasionally play a role. The perianth is reduced to two scales, called lodicules,:11 that expand and contract to spread the lemma and palea; these are generally interpreted to be modified sepals. This complex structure can be seen in the image on the right, portraying a wheat (Triticum aestivum) spikelet. The fruit of grasses is a caryopsis, in which the seed coat is fused to the fruit wall.:16 A tiller is a leafy shoot other than the first shoot produced from the seed.:11
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.:113–114
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, which allows for increased water use efficiency, rendering them better adapted to hot, arid environments and those lacking in carbon dioxide.
The C3 grasses are referred to as "cool-season" grasses, while the C4 plants are considered "warm-season" grasses.:18–19
The grass family is one of the most widely distributed and abundant groups of plants on Earth. Grasses are found on every continent, including Antarctica with the presence of Antarctic hair grass on the Antarctic Peninsula.
Grasses are the dominant vegetation in many habitats, including grassland, salt-marsh, reedswamp and steppes. They also occur as a smaller part of the vegetation in almost every other terrestrial habitat. Grass-dominated biomes are called grasslands. If only large, contiguous areas of grasslands are counted, these biomes cover 31% of the planet's land. Grasslands include pampas, steppes, and prairies. Grasses provide food to many grazing mammals—such as livestock, deer, and elephants—as well as to many species of butterflies and moths. Many types of animals eat grass as their main source of food, and are called graminivores – these include cattle, sheep, horses, rabbits and many invertebrates, such as grasshoppers and the caterpillars of many brown butterflies. Grasses are also eaten by omnivorous or even occasionally by primarily carnivorous animals.
Grasses are unusual in that the meristem is located near the bottom of the plant; hence, they can quickly recover from cropping at the top. The evolution of large grazing animals in the Cenozoic contributed to the spread of grasses. Without large grazers, fire-cleared areas are quickly colonized by grasses, and with enough rain, tree seedlings. Trees eventually outcompete most grasses. Trampling grazers kill seedling trees but not grasses.:137
Grasses are, in human terms, perhaps the most economically important plant family. Their economic importance stems from several areas, including food production, industry, and lawns. They have been grown as food for domesticated animals for up to 6,000 years and the grains of grasses such as wheat, rice, maize (corn) and barley have been the most important human food crops. Grasses are also used in the manufacture of thatch, paper, fuel, clothing, insulation, timber for fencing, furniture, scaffolding and construction materials, floor matting, sports turf and baskets.
Agricultural grasses grown for their edible seeds are called cereals or grains (although the latter term, agriculturally, refers to both cereals and legumes). Of all crops grown, 70% are grasses. Three cereals—rice, wheat, and maize (corn)—provide more than half of all calories consumed by humans. Cereals constitute the major source of carbohydrates for humans and perhaps the major source of protein, including 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.
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.
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, and for biofuel production.Bamboo scaffolding is able to withstand typhoon-force winds that would break steel scaffolding. 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.
Phragmites australis (common reed) is important for thatching and grass roots stabilize the sod of sod houses. Reeds are used in water treatment systems, in wetland conservation and land reclamation in Afro-Eurasia. Marram grass (Ammophila arenaria)
Grasses are the primary plant used in lawns, which themselves derive from grazed grasslands in Europe. They also provide an important means of erosion control (e.g., along roadsides), especially on sloping land. 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, seed heads. They are often used in natural landscaping, xeriscaping and slope stabilization in contemporary landscaping, wildlife gardening, and native plant gardening.
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.
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.
Grass on golf courses is kept in three distinct conditions: that of the rough, the fairway, and the putting green. Grass on the fairway is mown short and even, allowing the player to strike the ball cleanly. Playing from the rough is a disadvantage because the long grass may affect the flight of the ball. Grass on the putting green is the shortest and most even, ideally allowing the ball to roll smoothly over the surface. An entire industry revolves around the development and marketing of grass varieties for golf courses.
In tennis, grass is grown on very hard-packed soil, and the bounce of a tennis ball may vary depending on the grass's health, how recently it has been mowed, and the wear and tear of recent play. The surface is softer than hard courts and clay (other tennis surfaces), so the ball bounces lower, and players must reach the ball faster resulting in a different style of play which may suit some players more than others. Among the world's most prestigious court for grass tennis is Centre Court at Wimbledon, London which hosts the final of the annual Wimbledon Championships in England, one of the four Grand Slam tournaments.
Grasses have long had significance in human society. They have been cultivated as feed for people and domesticated animals for thousands of years. The primary ingredient of beer is usually barley or wheat, both of which have been used for this purpose for over 4,000 years.
In some places, particularly in suburban areas, the maintenance of a grass lawn is a sign of a homeowner's responsibility to the overall appearance of their neighborhood. One work credits lawn maintenance to:
...the desire for upward mobility and its manifestation in the lawn. As Virginia Jenkins, author of The Lawn, put it quite bluntly, 'Upper middle-class Americans emulated aristocratic society with their own small, semi-rural estates.' In general, the lawn was one of the primary selling points of these new suburban homes, as it shifted social class designations from the equity and ubiquity of urban homes connected to the streets with the upper-middle class designation of a "healthy" green space and the status symbol that is the front lawn.
Many US municipalities and homeowners' associations have rules which require lawns to be maintained to certain specifications, sanctioning those who allow the grass to grow too long. In communities with drought problems, watering of lawns may be restricted to certain times of day or days of the week.
Some common aphorisms involve grass. For example:
A folk myth about grass is that it refuses to grow where any violent death has occurred.
The Andropogoneae, sometimes called the sorghum tribe, are a large tribe of grasses (family Poaceae) with roughly 1,200 species in 90 genera, mainly distributed in tropical and subtropical areas. They include such important crops as maize (corn), sugarcane, and sorghum. All species in this tribe use C4 carbon fixation, which makes them competitive under warm, high-light conditions.Andropogoneae is classified in supertribe Andropogonodae together with its sister group Arundinelleae. Subdivisions include 12 subtribes, but the position of several genera within them is still unresolved (incertae sedis). Hybridisation was probably important in the evolution of the Andropogoneae, and the tribe's systematics is still not completely resolved.Aristideae
The Aristideae is the sole tribe of grasses in the monotypic subfamily Aristidoideae of the true grass family Poaceae. Its members are herbaceous annuals or perennials found in the tropics, subtropics and temperate zones. The tribe has over 300 species in three genera: The subfamily is a member of the PACMAD clade of grasses, the evolutionary group in which C4 photosynthesis independently evolved a number of times.
Arundinarieae is a tribe of bamboo in the grass family (Poaceae), placed in its own supertribe Arundinarodae and containing a single subtribe, Arundinariinae, and 30 genera. These woody bamboos occur in areas with warm temperate climates in southeastern North America, Subsaharan Africa, South Asia and East Asia. The tribe forms a lineage independent of the tropical woody bamboos (Bambuseae) and the tropical herbaceous bamboos (Olyreae).Bambuseae
The Bambuseae are the most diverse tribe of bamboos in the grass family (Poaceae). It consists of woody species from tropical regions, including some giant bamboos. Their sister group are the small herbaceous bamboos from the tropics in tribe Olyreae, while the temperate woody bamboos (Arundinarieae) are more distantly related. The Bambuseae fall into two clades, corresponding to species from the Neotropics (subtribes Arthrostylidiinae, Chusqueinae, and Guaduinae) and from the Paleotropics (subtribes Bambusinae, Hickeliinae, Melocanninae, and Racemobambosinae).The 68 genera are placed in seven subtribes:
Cephalostachyum, Davidsea, Dendrochloa, Melocanna, Neohouzeaua, Ochlandra, Pseudostachyum, Schizostachyum, Stapletonia, Teinostachyum
Cathariostachys, Decaryochloa, Hickelia, Hitchcockella, Nastus, Perrierbambus, Sirochloa, Valiha
Bambusa, Bonia, Cyrtochloa, Dendrocalamus, Dinochloa, Fimbribambusa, Gigantochloa, Greslania, Holttumochloa, Kinabaluchloa, Maclurochloa, Melocalamus, Mullerochloa, Nianhochloa, Neololeba, Neomicrocalamus, Oreobambos, Oxytenanthera, Parabambusa, Phuphanochloa, Pinga, Pseudobambusa, Pseudoxytenanthera, Soejatmia, Sphaerobambos, Temburongia, Temochloa, Thyrsostachys, Vietanamosasa
Apoclada, Eremocaulon, Guadua, Olmeca, Otatea
Actinocladum, Alvimia, Arthrostylidium, Athroostachys, Atractantha, Aulonemia, Cambajuva, Didymogonyx, Elytrostachys, Filgueirasia, Glaziophyton, Merostachys, Myriocladus, RhipidocladumChloridoideae
Chloridoideae is one of the largest subfamilies of grasses, with roughly 130 genera and 1,600 species, mainly found in arid tropical or subtropical grasslands. Within the PACMAD clade, their sister group are the Danthonioideae. The subfamily includes widespread weeds such as Bermuda grass (Cynodon dactylon) or goosegrass (Eleusine indica), but also millet species grown in some tropical regions, namely finger millet (Eleusine coracana) and teff (Eragrostis tef).
With the exception of some species in Ellisochloa and Eleusine indica, most of the subfamily's species use the C4 photosynthetic pathway. The first evolutionary transition from C3 to C4 photosynthesis in the grasses probably occurred in this subfamily, around 32 to 25 million years ago in the Oligocene.Festuca
Festuca (fescue) is a genus of flowering plants belonging to the grass family, Poaceae (subfamily Pooideae). They are evergreen or herbaceous perennial tufted grasses with a height range of 10–200 cm (4–79 in) and a cosmopolitan distribution, occurring on every continent except Antarctica. The genus is closely related to ryegrass (Lolium), and recent evidence from phylogenetic studies using DNA sequencing of plant mitochondrial DNA shows that the genus lacks monophyly. As a result, plant taxonomists have moved several species, including the forage grasses tall fescue and meadow fescue, from the genus Festuca into the genus Lolium, or alternatively into the segregate genus Schedonorus.
Because the taxonomy is complex it is not clear how many true species belong to the genus, but estimates range from over 400 to over 500.Fescue pollen is a significant contributor to hay fever.Graminid clade
The graminid clade is a clade of plants in the order Poales uniting four families, of which the grasses (Poaceae) are the most species-rich. Its sister group is the restiid clade.Grassland
Grasslands are areas where the vegetation is dominated by grasses (Poaceae); however, sedge (Cyperaceae) and rush (Juncaceae) families can also be found along with variable proportions of legumes, like clover, and other herbs. Grasslands occur naturally on all continents except Antarctica. Grasslands are found in most ecoregions of the Earth. For example, there are five terrestrial ecoregion classifications (subdivisions) of the temperate grasslands, savannas, and shrublands biome (ecosystem), which is one of eight terrestrial ecozones of the Earth's surface.List of Poaceae genera
The true grasses (Poaceae) are one of the largest plant families, with around 12,000 species and roughly 800 genera. They contain, among others, the cereal crop species and other plants of economic importance, such as the bamboos, and several important weeds.
Grasses probably originated in the understory of tropical rainforests in the Late Cretaceous, but have since come to occupy a wide range of different habitats. Notably, they are the dominant species in grasslands, open habitats that cover around one fifth of the earth's terrestrial surface. The C4 photosynthetic pathway has evolved at least 22 times independently in the grasses; C4 species are more competitive than C3 plants in open habitats with high light intensity and warm temperatures.The deeper relationships in the family have been resolved by recent molecular phylogenetic work. This has been translated into a modern classification which divides the grasses into twelve subfamilies and a number of tribes, with large tribes further divided into subtribes.Anomochlooideae, Pharoideae and Puelioideae are early diverging lineages containing only a few species. Most of the diversity falls into the two big BOP and PACMAD clades, which each contain roughly half of the family's species. C4 lineages have only evolved in the PACMAD clade, whereas many lineages in the BOP clade have evolved adaptations to cold climate.
While the higher-level classification of the grasses is now relatively well understood, taxonomic efforts continue at the species and genera level, and with continuing phylogenetic research, a number of names is likely to change. The list of genera below is therefore likely to evolve with further study.Panicoideae
Panicoideae is the second-largest subfamily of the grasses with over 3,500 species, mainly distributed in warm temperate and tropical regions. It comprises some important agricultural crops, including sugarcane, maize (or corn), sorghum, and switchgrass.
C4 photosynthesis evolved independently a number of times in the subfamily, which presumably had a C3 ancestor.Poa
Poa is a genus of about 500 species of grasses, native to the temperate regions of both hemispheres. Common names include meadow-grass (mainly in Europe and Asia), bluegrass (mainly in North America), tussock (some New Zealand species), and speargrass. Poa (πόα) is Greek for "fodder". Poa are members of the Pooideae subfamily of the Poaceae family.Bluegrass, which has green leaves, derives its name from the seed heads, which are blue when the plant is allowed to grow to its natural height of two to three feet (0.6 to 0.9 meters).The genus Poa includes both annual and perennial species. Most are monoecious, but a few are dioecious (separate male and female plants). The leaves are narrow, folded or flat, sometimes bristled, and with the basal sheath flattened or sometimes thickened, with a blunt or hooded apex and membranaceous ligule.Poeae
The Poeae are the largest tribe of the grasses, with around 2,800 species in 118 genera. The tribe includes many lawn and pasture grasses.Pooideae
The Pooideae are the largest subfamily of the grass family Poaceae, with over 4,200 species in 14 tribes and roughly 200 genera. They include some major cereals such as wheat, barley, oat, rye and many lawn and pasture grasses. They are often referred to as cool-season grasses, because they are distributed in temperate climates. All of them use the C3 photosynthetic pathway.
The Pooideae are the sister group of the bamboos within the BOP clade, and are themselves subdivided into 14 tribes.Sorghum
Sorghum is a genus of flowering plants in the grass family Poaceae. Seventeen of the 25 species are native to Australia, with the range of some extending to Africa, Asia, Mesoamerica, and certain islands in the Indian and Pacific Oceans. One species is grown for grain, while many others are used as fodder plants, either cultivated in warm climates worldwide or naturalized, in pasture lands. Sorghum is in the subfamily Panicoideae and the tribe Andropogoneae (the tribe of big bluestem and sugarcane).Spikelet
A spikelet, in botany, describes the typical arrangement of grass flowers.
Each spikelet has one or more florets. The spikelets are further grouped into panicles or spikes. The part of the spikelet that bears the florets is called the rachilla. A spikelet consists of two (or sometimes fewer) bracts at the base, called glumes, followed by one or more florets. A floret consists of the flower surrounded by two bracts, one external—the lemma—and one internal—the palea. The perianth is reduced to two scales, called lodicules, that expand and contract to spread the lemma and palea; these are generally interpreted to be modified sepals.
The flowers are usually hermaphroditic—maize being an important exception—and mainly anemophilous or wind-pollinated, although insects occasionally play a role.Triodia (plant)
Triodia is a large genus of hummock-forming bunchgrass endemic to Australia. They are known by the common name spinifex, although they are not a part of the coastal genus Spinifex. Many of the soft-leaved members of this species were formerly included in the genus Plectrachne.Tussock (grass)
Tussock grasses or bunch grasses are a group of grass species in the Poaceae family. They usually grow as singular plants in clumps, tufts, hummocks, or bunches, rather than forming a sod or lawn, in meadows, grasslands, and prairies. As perennial plants, most species live more than one season. Tussock grasses are often found as forage in pastures and ornamental grasses in gardens.Many species have long roots that may reach 2 meters (6.6 ft) or more into the soil, which can aid slope stabilization, erosion control, and soil porosity for precipitation absorption. Also, their roots can reach moisture more deeply than other grasses and annual plants during seasonal or climatic droughts. The plants provide habitat and food for insects (including Lepidoptera), birds, small animals and larger herbivores, and support beneficial soil mycorrhiza. The leaves supply material, such as for basket weaving, for indigenous peoples and contemporary artists.
Tussock and bunch grasses occur in almost any habitat where other grasses are found, including: grasslands, savannas and prairies, wetlands and estuaries, riparian zones, shrublands and scrublands, woodlands and forests, montane and alpine zones, tundra and dunes, and deserts.
Subfamilies and tribes of the grasses (Poaceae)
see also: List of Poaceae genera