Poaceae

? Poaceae (true grasses)
Flowering head of Meadow Foxtail (Alopecurus pratensis), with stamens exserted at anthesis
Plant Info
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

The family Poaceae, in the Class Liliopsida of the flowering plants, is also known as Gramineae. 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".

Structure and growth

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

General aspects

Structure of a grass plant.

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 regularly without damage to the growing point.

Reproduction

Parts of a spikelet

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 [[ rhizome|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.

Grass evolution

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, 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.

Cultivation and uses

Agricultural grasses grown for their edible seeds are called cereals. 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. Some other grasses are of major importance for foliage production. 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.

Grasses are used for construction; 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. Grasses are the primary plant used in lawns, which themselves derive from grazed grasslands in Europe. Phragmites australis (common reed) is important in water treatment, wetland habitat preservation and land reclamation in the Old World.

Grasses are used as food plants by many species of butterflies and moths. see List of Lepidoptera which feed on grasses.

Economically important grasses

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 Zoysia

Model organisms Brachypodium distachyon Maize (Corn) Rice Sorghum               Wheat

Grass and society

Grass covered house in Iceland

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 idioms evoke images of grass. For example:

• "The grass is always greener on the oth

er side" suggests that the greenness of grass is a positive quality.

• "Don't let the grass grow under your feet" references the speed with which grass grows.
• "A snake in the grass" cautions about the dangers that may be hidden in the grass.
• "When elephants fight, it is the grass who suffers" personifies the grass.

Image gallery

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  Leaves of Poa trivialis showing the ligules

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  Bamboo stem and leaves, nodes are evident

• 

  Wheat spike and spikelet

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  Spikelet opened to show caryopsis

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  Harestail Grass

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  Grass

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  Sugarcane (Saccharum officinarum)

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  Roots of Bromus hordeaceus

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  Barley mature spikes (Hordeum vulgare)

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  Illustration depicting both staminate and pistillate flowers of maize (Zea mays)

See also

• agrostology
• grass
• sedges

External links and references

• 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