Domestication of poaceous cereal crops such as maize (corn), wheat, rice, barley, and millet lies at the foundation of sedentary living and civilization around the world, and the Poaceae still constitute the most economically important plant family in modern times, providing forage, building materials (bamboo, thatch) and fuel (ethanol), as well as food.
Grasses generally have the following characteristics (the image gallery can be used for reference):
Poaceae have hollow stems called culms plugged at intervals by solid leaf-bearing nodes. Grass leaves are nearly always alternate and distichous (in one plane), and have parallel veins. Each leaf is differentiated into a lower sheath hugging the stem and a blade with entire (i.e., smooth) margins. The leaf blades of many grasses are hardened with silicaphytoliths, 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.
Parts of a spikelet
Flowers of Poaceae are characteristically arranged in spikelets, each spikelet having one or more florets. The spikelets are further grouped into panicles or spikes. 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 flowers are usually hermaphroditic—maize being an important exception—and anemophilous or wind-pollinated. 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. This complex structure can be seen in the image on the right, portraying a wheat (Triticum aestivum) spikelet.
The fruit of Poaceae is a caryopsis, in which the seed coat is fused to the fruit wall.
A tiller is a leafy shoot other than the first shoot produced from the seed.
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.
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; they may be either annual or perennial.
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.
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 shade out and kill most grasses. Trampling grazers kill seedling trees but not grasses.
Until recently, fossil findings indicated that grasses evolved around 55 million years ago. Recent findings of grass-like phytoliths in Cretaceousdinosaurcoprolites have pushed this date back to 66 million years ago. Indeed, revised dating of the origins of the rice tribe Oryzeae suggest a date as early as 107 to 129 Mya.
The relationships among the subfamilies Bambusoideae, Ehrhartoideae and Pooideae in the BEP clade have been resolved: Bambusoideae and Pooideae are more closely related to each other than to Ehrhartoideae. This separation occurred within a relatively short time span (about 4 million years).
The grass family is one of the most widely distributed and abundant groups of plants on Earth. Grasses are found on every continent, and are absent only from central Greenland and much of Antarctica.
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)
The Poaceae name was given by John Hendley Barnhart in 1895, based on the tribe Poeae (described in 1814 by Robert Brown), and the type genus Poa (described in 1753 by Linnaeus). The term is derived from the Ancient Greek term for grass.
Sugarcane is the major source of sugar production. Many other grasses are grown for forage and fodder for animal feed, particularly for sheep and cattle, thereby indirectly providing more human calories.
Grasses are used for construction. Scaffolding made from bamboo 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, and grass roots stabilize the sod of sod houses. Arundo is used to make reeds for woodwind instruments, and bamboo is used for innumerable implements.
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.
Grasses have long had significance in human society. They have been cultivated as feed for domesticated animals for up to 10,000 years, and have been used to make paper since the second century AD. Also, the primary ingredient of beer is usually barley or wheat, both of which have been used for this purpose for over 4,000 years.
^Prasad, V.; Stroemberg, C.A.E.; Alimohammadian, H.; Sahni, A. (2005). "Dinosaur Coprolites and the Early Evolution of Grasses and Grazers". Science(Washington) 310 (5751): 1177–1180. doi:10.1126/science.1118806. PMID16293759.Cite uses deprecated parameters (help)
^Prasad V, Strömberg CA, Leaché AD, Samant B, Patnaik R, Tang L, Mohabey DM, Ge S, Sahni A. (2011). Late Cretaceous origin of the rice tribe provides evidence for early diversification in Poaceae. Nat Commun. 2:480. doi:10.1038/ncomms1482PMID 21934664
^Wu ZQ, Ge S (2011) The phylogeny of the BEP clade in grasses revisited: Evidence from the whole-genome sequences of chloroplasts. Mol Phylogenet Evol
The true grasses, family Poaceae (formerly Gramineae), is one of the most speciose plant families, comprising over 10,000 species with a Gondwanan origin approximated at about 80-100 million years ago (although there are fossil specimens that potentially push the origin earlier; Prasad et al. 2011; Vicentini et al. 2008; Stevens 2013).
Distributed world-wide, the true grasses are absent only in parts of Greenland and Antarctica, and are the most economically important group of monocots, as this family includes the true grains, pasture grasses, sugar cane, and bamboo. Species in this family have been domesticated for staple food crops (grains and sugar, for example), fodder for domesticated animals, biofuel, building materials, paper and ornamental landscaping, among other things. Grasslands cover at least 20% of the earth’s surface, although grasses also grow in biomes other than grasslands.
Grasses are primarily wind pollinated, most have dangling anthers. They have hollow stems and grow from the plant base, rather than the tip, as an evolutionary response to predation. Many also protect themselves from predation by secreting silica crystals in their leaves. There are two main kinds of grasses, cool-season (C3) and warm-season (C4) grasses, which are distinct in their means for fixing Carbon. The evolution of C4 fixation has arisen independently in 4 of the 12 currently recognized grass subfamilies; a combination of changes in paleoclimate including temperature, aridness, seasonality are thought to select for new origins of C4 lineages (Vicentini et al. 2008).
Genomic duplications are common in the true grasses, and thought to play important role in the evolution of the group as well as innovations leading to diversification of branches within Poaceae (for example, the evolution of flowers arranged as spikelets).
(The Plant List 2010; Prasad et al. 2011; Stevens 2013; Vicentini et al. 2008; Wikipedia 2013)
The Plant List (2010). Poaceae. Version 1. Retrieved December 16, 2013 from http://www.theplantlist.org/
Prasad V, Strömberg CA, Leaché AD, Samant B, Patnaik R, Tang L, Mohabey DM, Ge S, and Sahni A. 2011. Late Cretaceous origin of the rice tribe provides evidence for early diversification in Poaceae. Nat. Commun. 2:480. doi:10.1038/ncomms1482 PMID 21934664.
Stevens, P. F. (2013). Angiosperm Phylogeny Website. Version 13, November 2013 [and more or less continuously updated since]. Retrieved December 16, 2013 from http://www.mobot.org/MOBOT/Research/APweb/welcome.html#Famlarge.
Vicentini, A., Barber, J. C., Aliscioni, S. S., Giussani, L. M., & Kellogg, E. A. 2008. The age of the grasses and clusters of origins of C4 photosynthesis. Global Change Biol. 14: 1-15.
Wikipedia, The Free Encyclopedia. 13 December, 2013. Poaceae. Retrieved December 16 2013 from http://en.wikipedia.org/w/index.php?title=Poaceae&oldid=585881309.
The following table shows the annual production of cereals in 1961, 2010, 2011, 2012, and 2013 ranked by 2013 production. All but buckwheat and quinoa are true grasses (these two are pseudocereals).
A staple food of people in the Americas, Africa, and of livestock worldwide; often called corn in North America, Australia, and New Zealand. A large portion of maize crops are grown for purposes other than human consumption. It can also be used for indirect human consumption through the production of the Mexican truffle.
The primary cereal of tropical and some temperate regions. Staple food in most of Brazil (both maize and manioc/cassava were once more important and its presence is still stronger in some areas), other parts of Latin America and some other Portuguese-descended cultures, parts of Africa (even more before the Columbian exchange), most of South Asia and the Far East. Largely overridden by breadfruit (a dicot tree) during the South Pacific's part of the Austronesian expansion.
The primary cereal of temperate regions. It has a worldwide consumption but it is a staple food of North America, Europe, Australia, New Zealand, most of the Southern Cone and much of the Greater Middle East. Wheat gluten-based meat substitutes are important in the Far East (albeit less than tofu) and said to resemble meat texture more than others.
Formerly the staple food of Scotland and popular worldwide as a winter breakfast food and livestock feed. Processed oatmeal in Latin America is often consumed as breakfast/tea/desserts year-round added to bananas (often soaked in previously smashed raw ones) in more gluten-avoiding (like cheese buns) and/or exercise-intensive diets.
Pseudocereal, traditional to the Andes, but increasingly popular elsewhere.
Maize, wheat, and rice together accounted for 89% of all cereal production worldwide in 2012, and 43% of all food calories in 2009, while the production of oats and triticale have drastically fallen from their 1960s levels. Other grains that are important in some places, but that have little production globally (and are not included in FAO statistics), include:
Teff, an ancient grain that is a staple in Ethiopia. It is high in fiber and protein. Its flour is often used to make injera. It can also be eaten as a warm breakfast cereal similar to farina with a chocolate or nutty flavor. Its flour and whole grain products can usually be found in natural foods stores.
Wild rice, grown in small amounts in North America.
Amaranth, an ancient pseudocereal, formerly a staple crop of the Aztec Empire and now widely grown in Africa.
While each individual species has its own peculiarities, the cultivation of all cereal crops is similar. Most are annual plants; consequently one planting yields one harvest. Wheat, rye, triticale, oats, barley, and spelt are the "cool-season" cereals. These are hardy plants that grow well in moderate weather and cease to grow in hot weather (approximately 30 °C, but this varies by species and variety). The "warm-season" cereals are tender and prefer hot weather. Barley and rye are the hardiest cereals, able to overwinter in the subarctic and Siberia. Many cool-season cereals are grown in the tropics. However, some are only grown in cooler highlands, where it may be possible to grow multiple crops in a year.
For a few decades, there has also been increasing interest in perennial grain plants. This interest developed due to advantages in erosion control, reduced need of fertiliser, and potential lowered costs to the farmer. Though research is still in early stages, The Land Institute in Salina, Kansas has been able to create a few cultivars that produce a fairly good crop yield.
The warm-season cereals are grown in tropical lowlands year-round and in temperate climates during the frost-free season. Rice is commonly grown in flooded fields, though some strains are grown on dry land. Other warm climate cereals, such as sorghum, are adapted to arid conditions.
Cool-season cereals are well-adapted to temperate climates. Most varieties of a particular species are either winter or spring types. Winter varieties are sown in the autumn, germinate and grow vegetatively, then become dormant during winter. They resume growing in the springtime and mature in late spring or early summer. This cultivation system makes optimal use of water and frees the land for another crop early in the growing season.
Winter varieties do not flower until springtime because they require vernalization: exposure to low temperatures for a genetically determined length of time. Where winters are too warm for vernalization or exceed the hardiness of the crop (which varies by species and variety), farmers grow spring varieties. Spring cereals are planted in early springtime and mature later that same summer, without vernalization. Spring cereals typically require more irrigation and yield less than winter cereals.
Once the cereal plants have grown their seeds, they have completed their life cycle. The plants die and become brown and dry. As soon as the parent plants and their seed kernels are reasonably dry, harvest can begin.
In developed countries, cereal crops are universally machine-harvested, typically using a combine harvester, which cuts, threshes, and winnows the grain during a single pass across the field. In developing countries, a variety of harvesting methods are in use, depending on the cost of labor, from combines to hand tools such as the scythe or cradle.
If a crop is harvested during wet weather, the grain may not dry adequately in the field to prevent spoilage during its storage. In this case, the grain is sent to a dehydrating facility, where artificial heat dries it.
In North America, farmers commonly deliver their newly harvested grain to a grain elevator, a large storage facility that consolidates the crops of many farmers. The farmer may sell the grain at the time of delivery or maintain ownership of a share of grain in the pool for later sale. Storage facilities should be protected from small grain pests, rodents and birds.
Some grains are deficient in the essential amino acidlysine. That is why many vegetarian cultures, in order to get a balanced diet, combine their diet of grains with legumes. Many legumes, on the other hand, are deficient in the essential amino acid methionine, which grains contain. Thus, a combination of legumes with grains forms a well-balanced diet for vegetarians. Common examples of such combinations are dal (lentils) with rice by South Indians and Bengalis, dal with wheat in Pakistan and North India, and beans with corn tortillas, tofu with rice, and peanut butter with wheat bread (as sandwiches) in several other cultures, including Americans. The amount of crude protein found in grain is measured as the grain crude protein concentration.