Poaceae (Gramineae)
Paul M Peterson, Smithsonian Institution, Washington DC, USA
The grass family (Poaceae or Gramineae) is the fourth
largest flowering plant family and contains approxi-
mately11000 species innearly 800generaworldwide.We
currently recognise 12 subfamilies: Anomochlooideae,
Pharoideae, Puelioideae, Bambusoideae, Ehrhartoideae,
Pooideae, Aristidoideae, Panicoideae, Arundinoideae,
Micrairoideae, Danthonioideae and Chloridoideae, and in
these subfamilieswe recognise 50 tribes and81 subtribes.
Grasses are well adapted to open, marginal and fre-
quently disturbed habitats, and can be found on every
continent, including Antarctica. A grass is characterised
by having a caryopsis or grain, and the primary inflores-
cence is referred to as a spikelet with a lemma and palea.
The incorporation of two photosynthetic or carbon
dioxide assimilation pathways has led to the family’s
ability to occupy 31243%of the Earth’s surface in various
climatic environments as the dominant component, the
grasslands.
Economically Important Species
The grasses are the most important plant family for food
production. Rice (Oryza sativa L.), wheat (Triticum aesti-
vum L.), corn (Zea mays L.), barley (Hordeum vulgare L.),
rye (Secale cereale L.), oats (Avena sativa L.), sorghum
(Sorghum bicolour (L.) Moench), pearl millet (Pennisetum
glaucum (L.) R. Br.), finger millet (Eleusine coracana (L.)
Gaertn.) and tef (Eragrostis tef (Zucc.) Trotter) are widely
cultivated grains. Grains were one of the first plants
domesticated by humans and are the basis for all early
civilisations. Corn or maize originated in Mexico and
formed the basis for the Aztec, Inca and Mayan civilisa-
tions. Wheat and barley originated in the Middle East or
Fertile Crescent and were used to make breads, pastas and
beer.Rice originated in southeasternAsia and canbe called
the world’s most important crop, because it is estimated
that over 200million tonnes of rice are consumed each year
by 1.6 billion people. In wheat, two proteins, gliadin and
glutein, along with starch combine to form gluten. When
these proteins in bread flour are mixed with water and
kneaded, the resultant product is an elastic dough mixture
perfectly suited, with the addition of yeast, for baking. The
domestication of modern wheat, a hexaploid (6n=42), is
an interesting story in the coevolution of man and food,
because early domesticates of wheat, the einkorns (Triti-
cum monococcum L., 2n=14) and emmers (Triticum tur-
gidum L., 4n=28), were harder to harvest, that is, separate
the grains from florets called threshing, and when baked
produced inferior breads. Other notable economic uses of
grasses include landscaping, construction (primarily
bamboos), and sugar cane (Saccharum officinarum L.)
production. Using perennial grasses such as switchgrass
(Panicum virgatumL.) or giant miscanthus (Miscanthus 
giganteus Greef and Deuter ex Hodkinson and Renvoize),
for biofuel production is gaining popularity worldwide,
simply because burning renewable pelleted grasses lowers
airborne particulate matter in comparison to burning coal
andother fossil fuels.See also: Poales (Grasses); Starch and
StarchGranules; TriticumAestivumL (Wheat); ZeaMays
(Maize, Corn)
Morphology
The most important feature of this family is a one-seeded
indehiscent fruit (seed coat is fused with the ovary wall),
known as a caryopsis or grain (Figure 1). The grain is rich in
endosperm starch, although it can contain protein and
small quantities of fat. The embryo is located on the basal
portion of the caryopsis and contains high levels of protein,
fats and vitamins. The stems are referred to as culms and
the roots are fibrous, principally adventitious or arising
from lower portions of the culms. Silica is a conspicuous
component of the epidermis and stored in silica cells.Many
grasses have rhizomes (underground stems) or stolons
(horizontal above-ground branches) that allow for vege-
tative reproduction in perennial grasses. Another impor-
tant feature of grasses is intercalarymeristems,which allow
growth well below the apex, typically near the base of the
Introductory article
Article Contents
. Economically Important Species
. Morphology
. History and Phylogeny
. Ecology
Online posting date: 20th September 2013
eLS subject area: Plant Science
How to cite:
Peterson, Paul M (September 2013) Poaceae (Gramineae). In: eLS.
John Wiley & Sons, Ltd: Chichester.
DOI: 10.1002/9780470015902.a0003689.pub2
eLS & 2013, John Wiley & Sons, Ltd. www.els.net 1
plant. The leaves are parallel-veined and two-ranked, with
the basal portion forming cylindrical sheaths and the upper
portions referred to as a blade. A ligule, located on the
upper surface at the junction of the blade and sheath,
commonly consists of flaps of tissue or hairs. The primary
inflorescence is referred to as a spikelet with 1–many, two-
ranked bracts inserted along the floral axis or rachilla. The
lowest two bracts of each spikelet, inserted opposite each
other, are called glumes above which, along the rachilla,
are borne pairs of bracts termedflorets. Each floret consists
of a lemma (lower bract) and palea (upper bract). Within
each palea the highly reduced flowers can be found.
Because the morphological features are often cryptic or
lacking, identification to species is often very difficult and
requires a trained specialist. Each grass flower usually
consists of two or three small scales at the base called
lodicules, an ovary with a style and two plumose stigmas
and 1–6 but more commonly three stamens with basifixed
anthers that contain single-pored, wind-dispersed pollen
grains. Lodicules function to open the florets during
flowering and possibly represent reduced perianth (sepals
and petals) segments. See also: Silica
History and Phylogeny
The grass family was probably characterised as a distinct
entity in most cultures. Three hundred years before the
Figure 1 Diagnostic features of a grass, Festuca californica Vasey: caryopsis, culm, floret, flower and spikelet. Illustrated by Alice R. Tangerini.
eLS & 2013, John Wiley & Sons, Ltd. www.els.net2
Poaceae (Gramineae)
Christian era, Theophrastus, a Greek scholar, recognised
the grass family and began to teach his students the con-
cepts of plant morphology. The first scientific subdivision
of the family was made by Brown (1814), who recognised
two different spikelet types between Panicoideae and
Pooideae (Festucoideae) subfamilies. Bentham (1881)
recognised 13 tribes grouped in to two major subfamilies.
Hitchcock (1935) and Hitchcock and Chase (1951) in their
treatments of the grasses of the US, recognised 14 tribes in
these two major subfamilies. The two-subfamily classifi-
cationwasusedbymost agrostologists for almost 150 years
until more modern syntheses. With the infusion of mole-
cular data, our present concept and classification of the
grasses is changing at a rapid rate. We currently recognise
12 subfamilies: Anomochlooideae, Pharoideae, Puelioi-
deae, Bambusoideae, Ehrhartoideae, Pooideae, Aris-
tidoideae, Panicoideae, Arundinoideae, Danthonioideae
and Chloridoideae, and in these subfamilies we recognise
50 tribes and 81 subtribes (Barkworth et al., 2003, 2007;
Soreng et al., 2012). The crown age for the grasses has been
estimated to be 71+ 9 million years old (Vicentini et al.,
2008). See also: Brown, Robert
Ecology
The highly reduced floral structure and wind pollination in
the grasses have enabled the family to be extremely suc-
cessful in planet-wide radiation and colonisation. Grasses
are well adapted to open, marginal and frequently dis-
turbed habitats, and can be found on every continent,
including Antarctica. Two major photosynthetic CO2
assimilation pathways can be found in the grasses (C3-fix-
ing of CO2 by ribulose 1,5-diphosphate (Calvin–Benson
cycle found in all vascular plants)) and C4-fixing of CO2 by
an additional enzyme (phosphoenolpyruvate) to form four
carbon molecules (oxaloacetate or malate) and there are
anatomical, physiological, phytogeographical and ecolo-
gical differences between these two types. The C3 grasses
are well adapted to temperate climates with winter pre-
cipitation, whereas C4 grasses are well suited to tropical
environments with summer/autumn precipitation. The
addition of C4 photosynthesis has allowed the grasses to
outcompete other plants in warm, tropical environments
by lowering the oxidation levels (photorespiration) of
photosynthetic products. All of these features have led to
the family’s ability to occupy 31–43% of the Earth’s sur-
face in various climatic environments as the dominant
component, the grasslands (Gibson, 2009). Historically,
the grassland biome has been maintained by a myriad of
biotic, climatic and edaphic effects. First, there must be a
dry season in which grasses and adjacent forest border dry
out and become flammable. Repeated fires favour grasses
over most tree and shrub species, because they very easily
re-sprout from the base. Second, large herbivorous mam-
mals (e.g. bison, antelope, yaks, guanacos, vicun˜as and
llamas) are instrumental at maintaining and further
opening up grassland communities. An often overlooked
consequence of grazing animals is their effect on soil
compaction which again favours sod-forming grasses over
trees and shrubs. See also: Calvin, Melvin; Grasslands;
Grazer-dominated Ecosystems; Photosynthesis: Ecology;
Photosynthetic Carbon Metabolism
Although the grasses seem well suited to ecosystem
margins andmoderately disturbed sites, themajor threat to
extant species is loss of habitat. Grasses have been very
successful in an evolutionary sense and in their ability to
adapt to human needs as major food sources for all com-
plex civilisations. It is important for us to maintain and
manage these critical resources for future generations. One
way to accomplish this goal is to preserve as many of the
wild relatives as possible because theymay carry the genetic
code for improved production and/or pest resistance. Once
these genes are introduced into a crop species, the hybrid
vigour of a crop can be dramatically improved, thus
allowing increased pest resistance and productivity.
For planet Earth,Walter andGillett (1997) list 776 grass
species as threatened; this includes possibly extinct,
endangered, rare, vulnerable and indeterminate categories.
InNorth America there are 215 species listed as threatened
with 40 of these occurring in California (19%) alone
(Peterson and Soreng, 2007). In comparison, there are only
80 species listed as threatened in South America. South
America is only three-quarters the size of North America,
but there are approximately twice asmany threatened grass
species per square mile in North America as in South
America. Obviously, a major factor in determining the
status of these threatened grass species is the extent of
botanical knowledge that scientists have gathered on these
plants. We still need to learn more about the grasses in
South America.
References
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Poaceae (Gramineae)
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Further Reading
Axelrod DI (1985) Rise of the grassland biome, central North
America. Botanical Review 51: 163–201.
Chapman GP and Peat WE (1992) An Introduction to the Grasses
(Including Bamboos and Cereals). Wallingford: CAB
International.
Clayton WD, Vorontsova MS, Harman KT and Williamson H
(2006) GrassBase – The Online World Grass Flora. http://
www.rbgkew.org.uk/data/grasses-db.html.
Columbus JT, Friar EA, Porter JM, Prince LMand SimpsonMG
(eds) (2007) Monocots: Comparative Biology and Evolution 2
Poales Claremont: Allen Press.
Grass Phylogeny Working Group (GPWG) (2001) Phylogeny
and subfamilial classification of the grasses (Poaceae).Annals of
the Missouri Botanical Garden 88: 373–457.
Harrington HD (1977) How to Identify Grasses and Grasslike
Plants. Chicago: Swallow Press.
Jacobs WLJ and Everett J (eds) (2000) Grasses: Systematics and
Evolution. Collingwood: CSIRO.
Peterson PM, Romaschenko K and Johnson G (2010) A classi-
fication of the Chloridoideae (Poaceae) based on multi-gene
phylogenetic trees. Molecular Phylogenetics and Evolution 55:
5802598. http://dx.doi.org/10.1016/j.ympev.2010.01.018
Seberg O, Petersen G, Barfod AS and Davis JI (eds) (2010)
Diversity, Phylogeny, and Evolution in the Monocotyledons.
Denmark: Aarhus University Press.
Soreng RJ and Davis JI (1998) Phylogenetics and character evo-
lution in the grass family (Poaceae): simultaneous analysis of
morphological and chloroplast DNA restriction site character
sets. Botanical Review 64: 1–90.
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Poaceae (Gramineae)