Poaceae


Poaceae, also called Gramineae, is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as true grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and species cultivated in lawns and pasture. Poaceae is the most well-known family within the informal group known as grass.
With around 780 genera and around 12,000 species, the Poaceae is the fifth-largest plant family, following the Asteraceae, Orchidaceae, Fabaceae and Rubiaceae.
The Poaceae are the most economically important plant family, including staple foods from domesticated cereal crops such as maize, wheat, rice, oats, barley, and millet for people and as feed for meat-producing animals. They provide, through direct human consumption, a substantial portion of all dietary energy: rice provides 20%, wheat supplies 19%, and maize 5%. Some members of the Poaceae are used as building materials ; others can provide a source of biofuel, primarily via the conversion of maize to ethanol.
Grasses have stems that are hollow except at the nodes 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.
Grasslands such as savannah and prairie where grasses are dominant are estimated to constitute 40.5% of the land area of the Earth, excluding Greenland and Antarctica. Grasses are also an important part of the vegetation in many other habitats, including wetlands, forests and tundra.
Though they are commonly called "grasses", groups such as the seagrasses, rushes and sedges fall outside this family. The rushes and sedges are related to the Poaceae, being members of the order Poales, but the seagrasses are members of the order Alismatales. However, all of them belong to the monocot group of plants.

Description

Grasses may be annual or perennial herbs, generally with the following characteristics : The stems of grasses, called culms, are usually cylindrical and are hollow, plugged at the nodes, where the leaves are attached. Grass leaves are nearly always alternate and distichous, and have parallel veins. Each leaf is differentiated into a lower sheath hugging the stem and a blade with entire margins. The leaf blades of many grasses are hardened with silica phytoliths, 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.
Flowers of Poaceae are characteristically arranged in spikelets, each having one or more florets. The spikelets are further grouped into panicles or spikes. The part of the spikelet that bears the florets is called the rachilla. A spikelet consists of two 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 mainly anemophilous or wind-pollinated, although insects occasionally play a role. 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. The fruit of grasses 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.

Growth and development

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, stoloniferous, and rhizomatous.
The success of the grasses lies in part in their morphology and growth processes and in part in their physiological diversity. There are both C3 and C4 grasses, referring to the photosynthetic pathway 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.
The C3 grasses are referred to as "cool-season" grasses, while the C4 plants are considered "warm-season" grasses.
Although the C4 species are all in the PACMAD clade, it seems that various forms of C4 have arisen some twenty or more times, in various subfamilies or genera. In the Aristida genus for example, one species is C3 but the approximately 300 other species are C4. As another example, the whole tribe of Andropogoneae, which includes maize, sorghum, sugar cane, "Job's tears", and bluestem grasses, is C4. Around 46 percent of grass species are C4 plants.

Taxonomy

The name Poaceae 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 Carl Linnaeus. The term is derived from the Ancient Greek πόα.

Evolutionary history

Grasses include some of the most versatile plant life-forms. They became widespread toward the end of the Cretaceous period, and fossilized dinosaur dung belonging to the sauropod titanosaurs have been found containing phytoliths of a variety that include grasses that are related to modern rice and bamboo. Grasses have adapted to conditions in lush rain forests, dry deserts, cold mountains and even intertidal 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:
Before 2005, fossil findings indicated that grasses evolved around 55 million years ago. Finds of grass-like phytoliths in Cretaceous dinosaur coprolites from the latest Cretaceous aged Lameta Formation of India have pushed this date back to 66 million years ago. Due to high phosphatic 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. In 2011, fossils from the same deposit were found to belong to the modern rice tribe Oryzeae, suggesting substantial diversification of major lineages by this time.
In 2018, a study described grass microfossils extracted from the teeth of the hadrosauroid dinosaur Equijubus normani from northern China, dating to the Albian stage of the Early 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.
Fossils of Phragmites have been found in the Late Cretaceous of North America, particularly in the Maastrichtian aged Laramie Formation. However slightly older fossils of
Phragmites have been found in the Eastern coast of the US dating the Campanian.
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. This separation occurred within the relatively short time span of about 4 million years.
According to Lester Charles King, the spread of grasses in the Late Cenozoic would have changed patterns of hillslope evolution favouring slopes that are convex upslope and concave downslope and lacking a free 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 more soil creep.

Subdivisions

There are about 12,000 grass species in about 771 genera that are classified into 12 subfamilies. See the full list of Poaceae genera.
  • Anomochlooideae Pilg. ex Potztal, a small lineage of broad-leaved grasses that includes two genera
  • Pharoideae L.G.Clark & Judz., a small lineage of grasses of three genera, including Pharus and Leptaspis
  • Puelioideae L.G.Clark, M.Kobay., S.Mathews, Spangler & E.A.Kellogg, a small lineage of the African genus Puelia
  • Pooideae, including wheat, barley, oats, brome-grass, reed-grasses and many lawn and pasture grasses such as bluegrass
File:Setaria verticillata W IMG 1085.jpg|thumb|Setaria verticillata from Panicoideae
The grass family is one of the most widely distributed and abundant groups of plants on Earth. Grasses are found on every continent, including Antarctica. The Antarctic hair grass, Deschampsia antarctica is one of only two flowering plant species native to the western Antarctic Peninsula.

Ecology

Grasses are the dominant vegetation in many habitats, including grassland, salt-marsh, reedswamp and steppes. They also occur as a smaller part of the vegetation in almost every other terrestrial habitat.
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, as well as to many species of butterflies and moths.
Many types of animals eat grass as their main source of food, and are called graminivores – these include cattle, sheep, horses, rabbits and many invertebrates, such as grasshoppers and the caterpillars of many brown butterflies. Grasses are also eaten by omnivorous or even occasionally by primarily carnivorous animals.
Grasses dominate certain biomes, especially temperate grasslands, because many species are adapted to grazing and fire.
Grasses are unusual in that the meristem is near the bottom of the plant; hence, grasses can quickly recover from cropping at the top.
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 outcompete most grasses. Trampling grazers kill seedling trees but not grasses.