| Poaceae (true grasses) Temporal range: Late Cretaceous[1] - present |
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| Flowering head of Meadow Foxtail (Alopecurus pratensis), with stamens exerted at anthesis | |
| Scientific classification | |
| Kingdom: | Plantae |
| (unranked): | Angiosperms |
| (unranked): | Monocots |
| (unranked): | Commelinids |
| Order: | Poales |
| Family: | Poaceae Barnhart |
| Type genus | |
| Poa L. |
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| Subfamilies | |
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There are 12 subfamilies: |
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The Poaceae (also known as the Gramineae) is a large and nearly ubiquitous family of flowering plants. Members of this family are commonly called grasses, although the term "grass" is also applied to plants that are not in the Poaceae lineage, including the rushes (Juncaceae) and sedges (Cyperaceae). This broad and general use of the word "grass" has led to plants of the Poaceae often being called "true grasses". With over 10,025 currently accepted species, the Poaceae represent the fifth largest plant family. Only the Orchidaceae, Asteraceae, Fabaceae, and Rubiaceae have more species.[2]
Plant communities dominated by Poaceae are called grasslands; grasslands are estimated to comprise 20% of the vegetation cover of the Earth. Grass species also occur in many other habitats not formally considered to be grasslands, including different types of wetlands (e.g., fens, marshes), forests and tundra.
Poaceae are often considered to be the most important of all plant families to human economies: it includes the staple food grains and cereal crops grown around the world, lawn and forage grasses, and bamboo, which is widely used for construction throughout east Asia and sub-Saharan Africa. Civilization was founded largely on the ability to domesticate cereal grass crops around the world.
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Grasses generally have the following characteristics (the image gallery can be used for reference):
Poaceae have hollow stems called culms, which are plugged (solid) at intervals called nodes, the points along the culm at which leaves arise. Grass leaves are alternate, distichous (in one plane) or rarely spiral, and parallel-veined. Each leaf is differentiated into a lower sheath, which hugs the stem for a distance and a blade with margins usually entire. The leaf blades of many grasses are hardened with silica phytoliths, which helps discourage grazing animals. In some grasses (such as sword grass), this makes the edges of the grass blades 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.
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 called the lemma (the external one) and the palea (the internal). The flowers are usually hermaphroditic (maize, monoecious, is an exception) and pollination is always anemophilous, that is, by wind. 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 left, portraying a wheat (Triticum aestivum) spike.
The fruit of Poaceae is a caryopsis, in which the seed coat is fused to the fruit wall and thus, not separable from it (as in a maize kernel).
A tiller a non-seed leaf shoot.
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.[3]
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; they may be either annual or perennial.
Biomes dominated by grasses are called grasslands. If only large contiguous areas of grasslands are counted, these biomes cover 31% of the planet's land.[5] Grasslands go by various names depending on location, including pampas, plains, steppes, or prairie.
In addition to their use as forage worldwide by many grazing mammals, such as cattle and other livestock, deer, and elephants, grasses are used as food plants by many species of butterflies and moths; see List of Lepidoptera that feed on grasses.
The evolution of large grazing animals in the Cenozoic has contributed to the spread of grasses. Without large grazers, a clearcut of fire-destroyed area would soon be colonized by grasses and, if there is enough rain, tree seedlings. The tree seedlings would eventually produce shade, which kills most grasses. Large animals, however, trample the seedlings, killing the trees. Grasses persist because their lack of woody stems helps them to resist the damage of trampling.[6]
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,[1][7] may place the diversification of grasses to an earlier date. Indeed, revised dating of the origins of the rice tribe Oryzeae have been led to the suggestion that the date might be pushed back as early as 107 Ma to 129 Ma.[8]
The relationships among the subfamilies Bambusoideae, Ehrhartoideae and Pooideae in the BEP clade have been resolved: Bambusoideae and Pooideae are more closely related than Ehrhartoideae.[9] This separation occurred within a relatively short time span (~4 million years).
The grass family is one of the most widely distributed and abundant groups of plants on Earth. They are found on every continent, and are essentially only absent from central Greenland and much of Antarctica.[2]
The most recent classification of the grass family recognizes 12 subfamilies:
Depending on the classification followed, the family includes approximately 668 genera.[2]
The Poaceae was named by John Hendley Barnhart in 1895,[10] 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".
Grasses are, in human terms, perhaps the most economically important plant family. Grasses' economic importance stems from several areas, including food production, industry, and lawns.
Agricultural grasses grown for their edible seeds are called cereals. Three cereals – rice, wheat, and maize (corn) – provide more than half of all calories eaten by humans.[11] Of all crops, 70% are grasses.[12] Cereals constitute the major source of carbohydrates for humans and perhaps the major source of protein, and 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. 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.[5] 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.
Grass fiber can be used for making paper, and for biofuel production.
Phragmites australis (common reed) is important in water treatment, wetland habitat preservation and land reclamation in Afro-Eurasia.
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.
Although supplanted by artificial turf in some games, grasses are still an important covering of playing surfaces in many sports, including football, tennis, golf, cricket, and softball/baseball.
Ornamental grasses, such as perennial bunch grasses, are used in many styles of garden design for their foliage, inflorescences, seed heads, and slope stabilization. They are often used in natural landscaping, xeriscaping, contemporary or modern landscaping, wildlife gardening, and native plant gardening.
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Grasses have long had significance in human society. They have been cultivated as a food source 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.
Some common aphorisms involve grass. For example:
A folk myth about grass is that it refuses to grow where any violent death has occurred.[13]
See the full List of Poaceae genera.