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Poaceae (true grasses)
Flowering head of Meadow Foxtail (Alopecurus pratensis), with stamens exserted at anthesis
Scientific classification
Kingdom: Plantae Division: Magnoliophyta Class: Liliopsida Order: Poales Family: Poaceae (R.Br.) Barnhart
Subfamilies
There are 7 subfamilies: Subfamily Arundinoideae Subfamily Bambusoideae Subfamily Centothecoideae Subfamily Chloridoideae Subfamily Panicoideae Subfamily Pooideae Subfamily Stipoideae

Poaceae or Gramineae is a family in the Class Liliopsida of the flowering plants. Plants of this family are usually called grasses. There are about 600 genera and between 9,000–10,000 species of grasses (Kew Index of World Grass Species). Plant communities dominated by Poaceae are called grasslands; it is estimated that grasslands comprise 20% of the vegetation cover of the earth. This family is the most important of all plant families to human economies: it includes the staple food grains grown around the world, lawn and forage grasses, and bamboo, widely used for construction throughout Asia.

The term "grass" is also applied to many grass-like plants not in the Poaceae, leading to plants of the Poaceae often being called "true grasses".

Contents 1 Structure and growth 2 Grass evolution 3 Economic importance 3.1 Food production 3.2 Industry 3.3 Lawns 3.4 Economically important grasses 4 Ecological importance 5 Grass and society 6 Image gallery 7 See also 8 Notes 9 External links and references

Grasses generally have the following characteristics (it is advisable to have a look at the image gallery for reference):

General aspects

Poaceae have hollow stems called culms, plugged at intervals called nodes. Leaves are alternate, distichous (in one plane) or rarely spiral, parallel-veined and arise at the nodes. Each leaf is differentiated into a lower sheath hugging the stem for a distance and a blade with margin 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 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 to penetrate into the sheath.

Grass blades grow at the base of the blade and not from growing tips. This location of the grass growing point near the ground allows it to be grazed or mowed regularly without damage to the growing point.^[1]

Reproduction

Flowers of Poaceae are peculiar. They are typically arranged in a terminal panicle or spike made of many small spikelets, each spikelet having one or more florets (flowers). The florets are usually hermaphroditic (maize, monoecious, is an exception) and pollination is always anemophilous. The perianth is reduced. Each spikelet is protected by two (usually) bracts called glumes and each single floret is surrounded by two bracts called the lemma (the external one) and the palea (the internal). 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.

Grass plants also spread out from a parent plant. Growth habit describes the type of shoot growth present in particular grass plants and is directly related to their ability to spread out from the parent plant and ultimately form a clonal colony. There are three general classifications of growth habit present in grasses; bunch-type, stoloniferous, and rhizomatous.

The success of the grasses lies in part in their morphology and growth processes, and in part in their physiological diversity. 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 specialised Kranz leaf anatomy that particularly adapts them to hot climates and an atmosphere low in carbon dioxide.

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^[2], may place the diversification of grasses to an earlier date.

The flowers of grass are reduced from the general monocotyledon type. The immediate ancestor of the first grass may have been a small Liliaceous plant with rhizomes and many small flowers, growing in dense patches, which adopted wind pollination to escape limitations caused by shortage of insects to pollinate the flowers.

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.^[3] Of all crops, 70% are grasses.^[4] Cereals constitute the major source of carbohydrate 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 food, particularly for sheep and cattle. Some other grasses are of major importance for foliage production, thereby indirectly providing more human calories.

Grasses are used for construction. Scaffolding made from bamboo is 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 fibre 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 the Old World.

Grasses are the primary plant used in lawns, which themselves derive from grazed grasslands in Europe.

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.

Grain crops Barley Maize (Corn) Oats Rice Rye Sorghum               Wheat Millet

Leaf and stem crops         Bamboo Marram grass Meadow-grass Reed Ryegrass Sugarcane

Lawn grasses Bahia grass Bent grass Bermuda grass     Fescue Meadow-grass Ryegrass St. Augustine grass Zoysia

Model organisms Brachypodium distachyon Maize (Corn) Rice Sorghum               Wheat

The grass family is one of the largest plant families; according to Missouri Botanical Gardens, it has 10025 known species, making it fourth largest. It is definitely exceeded in size by the composite flower, orchid, and legume families. Poaceae may also be exceeded by Rubiaceae, which is fewer than 25 species behind grasses. As Rubiaceae is more than 2000 species ahead of the sixth largest family, the Lamiaceae, Poaceae is probably no smaller than fifth biggest overall.^[6]

Biomes dominated by grasses are called grasslands. If only large contiguous chunks 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.

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 or 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 they can survive trampling.^[7]

Grass has long had significance in human society. It has been cultivated as a food source for domesticated animals for up to 10,000 years, and has been used to make paper since at least as early as 2400 B.C.

Some common aphorisms involve grass. For example:

• "The grass is always greener on the other side" suggests that someone else's of something will always seem better.
• "Don't let the grass grow under your feet" tells someone to get moving.
• "A snake in the grass" means dangers that are hidden.
• "When elephants fight, it is the grass who suffers" tells of bystanders caught in the crossfire..

Leaves of Poa trivialis showing the ligules	Bamboo stem and leaves, nodes are evident	Wheat spike and spikelet	Spikelet opened to show caryopsis
Harestail Grass	Grass	Sugarcane (Saccharum officinarum)	Roots of Bromus hordeaceus
Barley mature spikes (Hordeum vulgare)	Illustration depicting both staminate and pistillate flowers of maize (Zea mays)

• agrostology
• grass
• sedges

• TurfFiles by North Carolina State University
• Kew Index of World Grass Species
• Definitions of Grass structures
• Poaceae in L. Watson and M. J. Dallwitz] (1992 onwards), The families of flowering plants.

Chapman, G.P. and W.E. Peat. 1992. An Introduction to the Grasses. CAB International, Wallingford.

Cheplick, G.P. 1998. Population Biology of Grasses. Cambridge Univ. Press, Cambridge.

• L. Watson and M. J. Dallwitz (1992 onwards). The grass genera of the world.
• Piperno, D. R. & Sues, H.-D. (2005). Paleontology: Dinosaurs Dined on Grass. Science 310 (5751): 1126-1128 (18 November 2005) summary.
• Interactive Keys to North American Grasses at Utah State University