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
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.
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
[1], 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
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 other 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
See also
★
agrostology
★
grass
★
sedges
References
1. Dinosaurs Dined on Grass Dolores R. Piperno and Hans-Dieter Sues
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
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