The Poaceae are the most economically important plant family, includingstaple foods from domesticatedcereal crops such asmaize,wheat,rice,oats,barley, andmillet for people and asfeed formeat-producing animals. They provide, through direct human consumption, a substantial portion of all dietary energy: rice provides 20%, wheat supplies 19%, and maize (corn) 5%.[6] Some members of the Poaceae are used as building materials (bamboo,thatch, andstraw); others can provide a source ofbiofuel, primarily via the conversion of maize toethanol.
Grasses havestems that are hollow except at thenodes and narrow alternate leaves borne in two ranks. The lower part of each leaf encloses the stem, forming a leaf-sheath. The leaf grows from the base of the blade, an adaptation allowing it to cope with frequent grazing.
Though they are commonly called "grasses", groups such as theseagrasses,rushes andsedges fall outside this family. The rushes and sedges are related to the Poaceae, being members of theorderPoales, but the seagrasses are members of the orderAlismatales. However, all of them belong to themonocot group of plants.
Description
Diagram of a typical lawn grass plant
Grasses may beannual orperennialherbs,[8]: 10 generally with the following characteristics (the image gallery can be used for reference): Thestems of grasses, calledculms, are usually cylindrical (more rarely flattened, but not 3-angled) and are hollow, plugged at thenodes, where the leaves are attached.[8][9] Grassleaves are nearly always alternate and distichous (in one plane), and have parallel veins.[8]: 11 Each leaf is differentiated into a lower sheath hugging the stem and a blade with entire (i.e., smooth) margins.[8]: 11 The leaf blades of many grasses are hardened withsilicaphytoliths, which discourage grazing animals; some, such assword grass, are sharp enough to cut human skin. A membranous appendage or fringe of hairs called theligule lies at the junction between sheath and blade, preventing water or insects from penetrating into the sheath.[8]: 11
Flowers of Poaceae are characteristically arranged inspikelets, each having one or more florets.[8]: 12 The spikelets are further grouped intopanicles 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, calledglumes, followed by one or more florets.[8]: 13 A floret consists of the flower surrounded by two bracts, one external—thelemma—and one internal—thepalea. The flowers are usuallyhermaphroditic—maize being an important exception—and mainlyanemophilous or wind-pollinated, although insects occasionally play a role.[10] Theperianth is reduced to two scales, calledlodicules,[8]: 11 that expand and contract to spread the lemma and palea; these are generally interpreted to be modified sepals. Thefruit of grasses is acaryopsis, in which the seed coat is fused to the fruit wall.[8]: 16 Atiller is a leafy shoot other than the first shoot produced from the seed.[8]: 11
Growth and development
Grass flowersIllustration depicting both staminate and pistillate flowers of maize (Zea mays)
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 begrazed ormown 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, andrhizomatous.[12]The success of the grasses lies in part in their morphology and growth processes and in part in their physiological diversity. There are bothC3 andC4 grasses, referring to the photosynthetic pathway for carbon fixation. The C4 grasses have a photosynthetic pathway, linked to specializedKranz leaf anatomy, which allows for increasedwater 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
Although the C4 species are all in thePACMAD clade (see diagram below), it seems that various forms of C4 have arisen some twenty or more times, in various subfamilies or genera. In theAristida genus for example, one species (A. longifolia) is C3 but the approximately 300 other species are C4. As another example, the whole tribe ofAndropogoneae, which includesmaize,sorghum,sugar cane, "Job's tears", andbluestem grasses, is C4.[14] Around 46 percent of grass species are C4 plants.[15]
Grasses include some of the most versatileplant life-forms. They became widespread toward the end of theCretaceous period, and fossilizeddinosaur dung (coprolites) belonging to thesauropodtitanosaurs (though this provenance has been questioned)[17] have been found containingphytoliths of a variety that include grasses that are related to modernrice andbamboo.[18] Grasses have adapted to conditions in lushrain forests, drydeserts, cold mountains and evenintertidal 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-likephytoliths inCretaceous dinosaurcoprolites from the latest Cretaceous (Maastrichtian) agedLameta Formation of India have pushed this date back to 66 million years ago.[19][20] Due to highphosphatic content of 12.2-16.2% in Type A coprolites collected from the Lameta, an omnivore is hypothesized to be the source, contradicting the hypothesis of a sauropod origin.[17] In 2011, fossils from the same deposit were found to belong to the modern rice tribeOryzeae, suggesting substantial diversification of major lineages by this time.[21]
In 2018, a study described grass microfossils extracted from the teeth of thehadrosauroiddinosaurEquijubus normani from northern China, dating to theAlbian stage of theEarly 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 ofPhragmites have been found in the Late Cretaceous of North America, particularly in the Maastrichtian agedLaramie Formation.[22] However slightly older fossils ofPhragmites have been found in the Eastern coast of the US dating theCampanian (such as in theBlack Creek Formation).[23]
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.[24] This separation occurred within the relatively short time span of about 4 million years.
According toLester Charles King, the spread of grasses in theLate Cenozoic would have changed patterns ofhillslope evolution favouring slopes that are convex upslope and concave downslope and lacking afree 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 moresoil creep.[25][26]
Subdivisions
There are about 12,000 grass species in about 771 genera that are classified into 12 subfamilies.[27] See the fulllist of Poaceae genera.
AnomochlooideaePilg. exPotztal, a small lineage of broad-leaved grasses that includes two genera (Anomochloa,Streptochaeta)
PharoideaeL.G.Clark &Judz., a small lineage of grasses of three genera, includingPharus andLeptaspis
Chloridoideae, including the lovegrasses (Eragrostis, about 350 species, includingteff), dropseeds (Sporobolus, some 160 species),finger millet (Eleusine coracana (L.) Gaertn.), and the muhly grasses (Muhlenbergia, about 175 species)
The grass family is one of the most widely distributed and abundant groups of plants onEarth. Grasses are found on every continent,[28][29] includingAntarctica. The Antarctic hair grass,Deschampsia antarctica is one of only two flowering plant species native to the westernAntarctic Peninsula.[30]
Ecology
Wind-blown grass in theValles Caldera in New Mexico, United States
Grasses dominate certainbiomes, especiallytemperate grasslands, because many species are adapted to grazing and fire.[36]
Grasses are unusual in that themeristem is near the bottom of the plant; hence, grasses can quickly recover from cropping at the top.[37]The evolution of large grazing animals in theCenozoic 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.[11]: 137
Of all crops grown, 70% are grasses.[40] Agricultural grasses grown for their edible seeds are calledcereals orgrains (although the latter term, when used agriculturally, refers to both cereals and similar seeds of other plant species, such asbuckwheat andlegumes). Three cereals—rice, wheat, and maize (corn)—provide more than half of all calories consumed by humans.[41] Cereals constitute the major source ofcarbohydrates for humans and perhaps the major source of protein; these include rice (insouthern andeastern Asia), maize (inCentral andSouth America), and wheat and barley (inEurope,northern Asia and theAmericas).
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 aspasture for foraging or asfodder for prescribedlivestock feeds, particularly in the case ofcattle,horses, andsheep. Such grasses may be cut and stored for later feeding, especially for the winter, in the form of bales ofhay orstraw, or in silos assilage. 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 ascob, for insulation, in the manufacture of paper and board such asoriented structural straw board. Grassfiber can be used for makingpaper,biofuel production,[42] nonwoven fabrics, and as replacement for glass fibers used in reinforced plastics.[43]Bamboo scaffolding is able to withstand typhoon-force winds that would break steel scaffolding.[31] Larger bamboos andArundo donax have stout culms that can be used in a manner similar to timber,Arundo is used to make reeds forwoodwind instruments, and bamboo is used for innumerable implements.[44]
Processed, fossilised pollen from the family Poaceae. Species unknown
Pollen morphology, particularly in thePoaceae family, is key to figuring out their evolutionary relationships andhow environments have changed over time.[48] Grass pollen grains, however, often look the same, making it hard to use them for detailed climate or environmental reconstructions.[49][50] Grass pollen has asingle 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 apartdomesticated grasses from wild ones,[51][52][53] and to indicate various biological features like how they performphotosynthesis,[54] their breeding systems,[54][55] and genetic complexity.[56][57][49] 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.[58][59][49] Despite these challenges, new techniques inFourier-Transform Infrared Spectroscopy and improved statistical methods are now helping to better identify these similar-looking pollen types.[50]
Grass playing fields, courses and pitches are the traditional playing surfaces for manysports, includingAmerican football,association football,baseball,cricket,golf, andrugby. Grass surfaces are also sometimes used forhorse racing andtennis. 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 withartificial turf, a synthetic grass-like substitute.[61]
Cricket
The gray area is thecricket pitch currently in use. Parallel to it are other pitches in various states of preparation which could be used in other matches.
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.[62]
Grass on golf courses is kept in three distinct conditions: that of therough, thefairway, and theputting 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 turf grass varieties.[63]
In tennis, grass is grown on very hard-packed soil, and the bounce of atennis ball may vary depending on the grass's health, how recently it has been mowed, and the wear and tear of recent play.[citation needed] The surface is softer thanhard courts andclay (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.[citation needed] Among the world's most prestigious court for grass tennis is Centre Court atWimbledon, London, which hosts the final of the annualWimbledon Championships in England, one of the fourGrand Slam tournaments.
A number of grasses are invasive species that damage natural ecosystems, including forms ofPhragmites australis which are native to Eurasia but has spread around the world.[64][65]
Role in society
Grass-covered house inIcelandTypical grass seen in meadowsGrass with non-grass flowers around it
Grasses have long had significance in human society. They have been cultivated as feed for people anddomesticated animals for thousands of years. The primary ingredient ofbeer is usually barley or wheat, each of which has been used for this purpose for over 4,000 years.[66]
In some places, particularly insuburban 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 ofThe 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.[67][68]
In communities withdrought problems, watering of lawns may berestricted to certain times of day or days of the week.[69] Many US municipalities and homeowners' associations have rules that require lawns to be maintained to certain specifications, sanctioning those who allow the grass to grow too long.[70][71]
^Berry, E. W. (1914). "The Upper Cretaceous and Eocene floras of South Carolina and Georgia".U.S. Geological Survey Professional Paper.84: 11.Bibcode:1914usgs.rept...11B.doi:10.3133/pp84.
^Soreng, Robert J.; Peterson, Paul M.; Romschenko, Konstantin; Davidse, Gerrit; Zuloaga, Fernando O.; Judziewicz, Emmet J.; Filgueiras, Tarciso S.; Davis, Jerrold I.; Morrone, Osvaldo (2015). "A worldwide phylogenetic classification of the Poaceae (Gramineae)".Journal of Systematics and Evolution.53 (2):117–137.Bibcode:2015JSyEv..53..117S.doi:10.1111/jse.12150.hdl:11336/25248.ISSN1674-4918.S2CID84052108.
^Matthew J. Lindstrom, Hugh Bartling,Suburban sprawl: culture, theory, and politics (2003), p. 72, quoting Virginia Scott Jenkins,The Lawn: A History of an American Obsession (1994), p. 21.
^"hexenal". School of Chemistry, University of Bristol.Archived from the original on 2013-10-09. Retrieved2013-08-26.
^Olmert, Michael (1996).Milton's Teeth and Ovid's Umbrella: Curiouser & Curiouser Adventures in History, p. 208. Simon & Schuster, New York.ISBN0-684-80164-7.
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