Introduction
1.Poaceae History:
Fossil evidence that Poaceae may have first appeared in the late
Cretaceous, approximately 70 million years ago ( Thomasson, 1987 ).
Although there are many fossil records for the grass family, the ambiguity
caused by their similarity to several related families such as Cyperaceae
and Juncaceae greatly reduces their application value. The extant grass
species share many unique characters in their stems, leaves, flowers and
inflorescences, and caryopsis fruits, and are placed in Liliopside
(Monocotyledonae) (Stebbins, 1982 & 1987). The monogeneric
Joinvilleaceae is though to be the most closely related family to Poaceae
,since Poaceae and Joinvilleaceae are phylogenetically allied and share a
very unique inverted repeat of about 6 kilobases in their chloroplast
genomes ( Doyle et al., 1992). The grass family was first named by de
Jussieu (1789), and the grouping of the 58 genera in his treatment was
mainly based on numerical characters, sush as number of styles, stamens
and florets. Brown 1814, the grass family was divided into " tribes":Trib
Paniceae with a basal reduction of the spikelets and Poaceae with an
apical reduction of the spikelets. Later, with additional evidence from leaf
epidermis and anatomy (Part, 1936; Brown,1958),chromosome number
and morphology (Avdulov,1931), embryo structure ( Reeder, 1959), and
numerical taxonomy ( Hilu and Wright, 1982; Watson et al.,1985), a
better understanding of grass systematic was achieved and various
subfamily systems were proposed. With the introduction of molecular
data such as from proteins and nucleic acids
more grouping patterns and evolutionary lineages of the grass family
were presented (Hamby and Zimmer,1988; Hilu and Esen, 1988; Doebley
et al.,1990; Davis and Soreng,1993; Cummings et al.,1994; Nadot et
al.,1994; ; Barker et al.,1995; Clark et al., 1995; Duvall and Morton,
1996; Hsiao et al.,1998). Although the number of subfamilies
recognized:Aundinoidea, Bambusoideae,Centothecoideae , Chloridoideae
, Oryzoideae, Paniccoideae and Pooideae.
2. Phylogeny and Classification of the Poaceae
2.1 Phylogeny of the Poaceae:
The Poaceae is classified within the Order Poales, which includes
the
Poaceae,Flagellariaceae,
Joinvillea,
Restionaceae,
Centrolepidaceae, Anarthriaceae,and Ecdeiocoleaceae. Within the
Poales, the Ecdeiocoleaceae is a sister to the Poaceae. These two
families share several features, namely a 6 kilobase (kb) inversion in the
chloroplast genome, and operculate, annulate pollen without scrobiculi
(Michelangeli et al., 2003).
Early attempts at intrafamilial classification systems divided the
Poaceae (then the Gramineae) into two subfamilies: the Paniceae, and a
large and heterogeneous Poaceae (Kellogg, 1998). This division was
based largely on gross morphological features of the inflorescence, and is
now known to be an artificial arrangement that does not reflect the
evolutionary history of the grass family. Examination of cytological,
micromorphological, physiological, and anatomical chararcters led to a
more natural system of classification ( Hilu and Wright, 1982). Molecular
data have also helped to clarify relationships within the grass family
(Davis and Soreng, 1993; Soreng and Davis, 1998; Mathews et al., 2000)
The most current description of subfamilies within the Poaceae was
proposed by The Grass Phylogeny Working Group (GPWG), 2001, based
on morphological, anatomical, cytological, and biochemical data, plus
nuclear and chloroplast DNA sequence data, and restriction site analysis.
The basal lineages comprise three subfamilies: the Pueloidea,Pharoideae,
and Anomochloideae.
Li & Ge,2001 studied Genetic variation and clonal diversity of seven
Psammochloa villosa (Poaceae) populations from North China by using
simple sequence repeat (ISSR) markers. Of the 84 primers screened, 12
produced highly reproducible ISSR bands. No significant differences in
genetic or clonal diversity were found between populations in mobile or
fixed dumes.
Xin-ming & Xiao-liang, 2005 analyzed of genetic relationships of
eight cultivars &lines in pennisetum by RAPD markers. There was also
genetic differentiation between the 'Hybridpennisetum' of the other seven
genotypes.
Song et al., 2006 estimated by ISSR assy variation in populations of
the cutgrass Leersia hexandra. The 12 used ISSR primers generated 175
bands. Significant genetic differentiation was found among populations.
This patter of genetic variation may be associated with reproductive mode
of its mixed breeding systems although mainly through vegetative
propagation.
Chandra & Saxena 2007 used molecular markers analyses including
identifying genes through association genetics approach requires DNA
from large numbers of samples of tropical grass species namely
Dichanthium annulatum, Hetreopogon concortus, Sehima nervosum,
Chrysopogon fulvus &Cenchrus glaucus is described .
Heikel et al.,2007 used Two types of molecular markers; random
amplified polymorphic DNA (RAPD) and inter-simple sequence repeat(
ISSR), were assayed to determine the genetic diversity of ten Gramineae
accessions from North Coast of Egypt. In RAPD analysis, 11 primers
initially screened displayed RAPD profiles with polymorphic bands, and
many of these bands varied in molecular weight, and intensity. In ISSR
analysis, 11 of tested ISSR primers generated variable banding patterns.
Genetic characterization not only provides databasc for genetic
biodiversity, but also is a necessity for the protection of Egyptian
royalties of the landraces.
El Rabey, 2008 invesitigated of genus Avena L. by using RAPD –
PCR& protein electrophoresis pattern was analysed using SDSpolyacrylamide gel electrophoresis under reducing conditions.
Vergara et al., 2008 certain Imperata morphotypes growing
in the filed in US are difficult to identify. To clarify their identity inter
simple sequence repeats (ISSRs) were used to asses genetic
differentiation among eight populations in the US.
Chandra & Dubey, 2010 used 187 decamer oligonucleotide primer
were tested for PCR-based DNA amplification of six prominent species
of genus Cenchrus in India.Of these, 32 potential repetitive of
polymorphic primer were tested for identification of species – specific
marker for C. ciliaris, C.setigerus, C. pennisetiformis, C. prieurri, C.
biflorus & C. myosuroides.
Haroun, 2010 described 15 proteins bands were identified in 23
accessions belong to 11 species of genus Panicum investigated belonging
three ploidy levels. The bands vary in number & positions between
accessions even within the same species.
Lin et al., 2010 identified two Bamboo species Phyllostachys
kwangsiensis & Phyllostachys bambusoides for the first time, and
obtained suspected Bamboo hybrids by using PCR / ISSR. They
concluded that ISSR markers are useful to identify Bamboo hybrids, and
that breeding between Bamboo species is possible & useful.
Lu et al., 2010 distinguished Apocynum venetum from Apocynum
pictum through the combinative technologies of bulked segregate analysis
( BSA )of randomly amplified polymorphic DNA (RAPD ).
Tamkoc & Arslan 2010 used storage proteins & agronomic traits for
investigate the genetic diversity among 11 Turkish Kentucky bluegrass
(Poa pratensis) genotypes using seed storage proteins and agronomic
traits. Thirty nine polypeptide bands in range of 10 to 82 kDa were
recorded. This clustering pattern for protein profiles & agronomic data
showed that same genotypes were revealed in same groups.
Vittal et al., 2010 analyzed based on their water – soluble protein &
prolamine – profiles. DNA – variation was investigated using twenty
microsatellite, or simple sequence repeat (SSR) markers, the data can be
used for identification of Sorghum bicolor L. Moench.
2.2 Origin and Evolution of the Poacea:
The date of the origin of grasses is controversial. Fossil evidence
suggests that the Poaceae likely originated in tropical areas during the
Late Cretaceous period, 70-55 million years ago (mya) (Kellogg, 2001).
Recent studies contradict this date of origin of the Poaceae. A molecular
clock estimates that grasses diverged approximately 83 mya (Janssen and
Bremer, 2004), while recently-discovered silica bodies of grasses in
fossilized dinosaur dung suggest that the BEP clade {Bambusoideae,
Ehrhtoideae and Pooideae} and PACCAD clades {Panicoideae,
Aristidoideae, Chloroideae, Centothecoideae, Arundioid and diverged
before 80 mya, much earlier than previously thought (Prasad et al.,
2005).
It is believed that grasses originated at the tropical forest margin,
from which the bamboos radiated into forest habitats, while other grasses
adapted to open habitats (Renvoize and Clayton, 1992).
A biogeographical study of the Poales suggests the ancestral area of the
grass family included South America, and possibly regions of Africa to
Australia (Bremer, 2002). Based on the present geographic distribution of
the basal lineages of the Poaceae, grasses may have arisen in
Gondwanaland (Clark et al., 1995), and they may have established their
current range through long-distance dispersal across the Indian and
Atlantic Oceans, or by vicariance through the breakup of the Gondwanan
continent. Grasses were not likely abundant in the Northern Hemisphere
until the Oligocene or Miocene era (Stebbins, 1987), and grass-dominated
ecosystems appeared in the mid-Miocene. Radiation into dry habitats,
facilitated by the evolution of drought tolerance, occurred millions of
years after the origin of the Poaceae, and is thought to be one of the
contributing factors to the abundance and diversity of grasses worldwide
(Kellogg, 2001).
The earliest grasses are thought to have been herbaceous,
rhizomatous, broad-leaved, wind-pollinated, and possessed six anthers
and three stigmas (Clark et al., 1995).
The caryopsis was present in the earliest grass lineages, but the
spikelet evolved in a series of steps after the origin of grasses (Kellogg,
2001). Early grasses had bracteate inflorescences, but lacked a true palea
and lemma, which evolved some time before the divergence of the
Pharoideae.
2.3 Morphological studies of the Poaceae:
Grasses have several distinctive morphological features. They are
the only plant group with paleas, lemmas, glumes, and a caryopsis as a
fruit type (GPWG 2001). Even though nearly all grasses share these
features, remarkable diversity of forms can be observed in virtually every
character. For example, grasses range in size from woody bamboos, such
as Dendrocalamus sinicus Chia et J.L. Sun that can attain a height of 30
m(Clayton et al., 2002 onwards), to Arctic grasses such as Phippsia
algida (Sol.)R. Br., which is only 2 to 15 cm high at maturity (Cody,
2000). In addition to a vast range of variability in stature, grasses have
adapted to a wide range of environmental conditions from obligate or
aquatics to desert species, which has also led to morphological variability.
To add even more complexity, many morphological characters have
evolved from multiple independent origins within the grass family, and
this homoplasy causes difficulty in reconstructing phylogenetic
relationships
in
the
Poaceae
(Stebbins
and
Crampton,1959).
Nonetheless, phenetic studies of the Poaceae, based on large
morphological data sets, have provided the basis for grass taxonomy (e.g.
Hilu and Wright, 1982; Watson and Dallwitz, 1992).
Morphological characters continue to play an important role in
studies of the Poaceae For enstance, arecent analysis of American species
of Tripogon Roem & Schult was based on leaf anatomy &
micromorphological features of spikelets (Rúgolo de Agrasar & Vega,
2004).
Inflorescence in the Poaceae are morphologically diverse, ranging
from spikes to many-branched panicles ( Doust and Kellogg, 2002a ).
Inflorescenes vary in the number of branches, the number of orders of
branching, and the degree of elongation of axes (Doust et al., 2005).
The majority of grasses have reproductive structures arranged into
florets, which are organized whithin a spikelet structure. The arrangement
of florets into spikelets that are subtended by sterile bract like glumes is
characteristic of the Spikelet Clade, which includes all of the grasses
except for the Anomochlooideae. Each floret typically possesses a lemma,
palea, lodicules, androecium, and gynoecium (GPWG 2001). Lemmas are
the sheath-like bracts lowest on the floret axis (Soreng and Davis,1998).
Paleas are considered prophylls, and are inserted above lemmas (GPWG
2001). Micromorphological traits of the palea and lemma have been
useful in recent phylogenetic studies of Bromus L. (Acedo and Llamas,
2001), and Melica L. (Mejia-Saules and Bisby, 2003). Lodicules are
considered homologous to petals in eudicots, and function to open florets
for fertilization (Bommert et al., 2005). Grasses florets typically possess
two lodicules, but a third lodicule, when present, is inserted higher on the
floral axis than the two anterior lodicules (Clifford,1987). The number,
shape, and vascularization of lodicules have traditionally been useful for
grass classification (Stebbins and Crampton, 1959). Vascularization of
lodicule is reduced or lost in the Pooideae and PACC clade of Soreng and
Davis (1998). Lodicule morphology may be informative in phylogenetic
studies, as Hsu (1965) indicated for Panicum L. Similarly, Melica L. and
Glyceria R.Br. share characteristic fusion of the anterior pair of lodicules
(GPWG 2001).
Early lineages of grasses have six stamens, therefore this is
considered the plesimorphic state in grasses. Thus the inner whorl of
stamens was lost before the divergence of the PACC clade and the
Pooideae (Soreng and Davis,1998), resulting in a reduction to three
stamens in most grasses. Reversal to six stamens has occurred in at least
three independent events in the Bambusoideae Ehrharrtoideae (GPWG
2001).
2.4 Morphology & Anatomy of Caryopsis:
The caryopsis is a unique fruit type, and it is a distinguishing
feature of the Poaceae (GPWG 2001). The caryopsis is similar to an
achene in that both are dry indehiscent fruits; however, the pericarp is
adnate to the seed coat in the caryopsis. Even though the pericarp is free
in some grasses, these fruits are not considered achenes, but rather the
free pericarp represents a modification of the caryopsis (Brandenburg,
2003). Other examples of variability in the pericarp (caryopsis surface) in
grasses include differences in textural patterns, including reticulate,
verrucate, striate, substriate, tuberculate, regulate, echinate, psilate,
lophate, and foveolate (Jordan et al., 1983).
Variability of surface texture of the caryopsis is also taxonomically
informative at specific and generic levels when examined with scanning
electron microscopy (SEM) (Sendulsky et al., 1987). Some grass genera
are clearly distinguished because all the species share a common
caryopsis surface pattern type, such as Digitaria Haller, which is
characterized by a verrucate caryopsis surface (Jordan et al., 1983).
Moreover, (Barker et al.,1995) described types of surface
patterns in seven Danthonioid genera, reporting the rugose type in
Karroochloa and Tribolium Desv., scalarifom – reticulate patterns in
Chaetobromus Nees, Pseudopentameris Conert, Merxmuellera Conert,
and species of Pentaschistis (Ness) Spach., and a deeply reticulate pattern
in other species of Pentaschistis. The colliculate type was documented
in Pentameris P. Beauv. A separate study reported
the straight
reticulate
type in Danthonia californica ( Jordan et al., 1983).
Additionally, caryopsis characters such as length and width of
epicarp cells, the degree of concavity of periclinal walls, and the shape of
cell wall undulations, are useful to differentiate among European species
of Echinochloa Beauv. (Costea and Tardif,2002), and species of
Eraggrostis
N.M.Wolf
in
Australia
(Lazarides,1997).Within
the
Choloridoideae, caryopsis traits, such as the ventral face and hilum
morphology are useful at the tribal and generic levels (Liu et al., 2005b).
Hilum is punctuate in Notodanthonia and linear in Danthonia (Zotov,
1963). Similarly, caryopsis size, shape, beak, and degree of pericarp
fusion are useful in differentiating North and South American species of
Diarrhena P. Beauv. (Brandenburg et al.,1991).
In general, caryopsis morphology and its associated features are
relatively poorly understood in grasses, but previous studies have
demonstrated its taxonomic utility. For instance, the obovoid caryopsis
shape is character shared by Rytidosperma and Danthonia, providing
evidence of a close phylogenetic relationship between these two genera
(Wright, 1984).
However, caryopsis micromorphology is poorly documented in
panicoid grasses, these characters may also be of taxonomic significance
in this group. The structure of transverse cells of the pericarp is useful for
identifying fossilized caryopsis of Triticum L. and Secale L. (KorberGrohne,1981). Furthermore, SEM examination of surface patterns on
fossilized caryopsis remnants has been important in determining the
evolution and origin of cultivation in crops such as Eleusine coracana (
L.) Gaertn. Subsp. Coracana ( finger millet ) ( Hilu et al., 1979).Thus,
ample evidence indicates that the surface pattern of the caryopsis and its
associated microstructure are informative.
In addition, caryopsis characters may represent an important link
to the fossil record for a better understanding of the conditions under
which cereals were domesticated.
Other characters are also taxonomically informative at the
subfamily level in the Poaceae, including embryo length relative to the
length of the caryopsis, and the shape of the hilum (Stebbins and
Crampton,1959). Embryos tend to be longer relative to the caryopsis
length in pooids, versus smaller embryos in panicoid grasses (Sendulsky
et al.,1987). The linear hilum is plesiomophic, and is characteristic of all
basal lineages (GPWG 2001). Short hila are characteristic of the PACC
clade, with reversals to the long, linear state in some taxa in the
Arundinoideae (Soreng and Davis,1998).
2.5 Leaf Anatomy:
Micromorphological characters of the leaf epidermis have been
taxonomically informative throughout the plant kingdom, and are
especially important for comparing extant taxa to relatives in the fossil
record (Stace, 1984). Morphological and anatomical descriptions of grass
leaf blades were compiled by Metcalfe (1960), including detailed
descriptions of the leaf epidermis across a broad, though incomplete,
taxonomic sampling. Several features of the leaf epidermis, including:
intercostal long and short cells, stomatal cell type and shape, type of
papillae, prickle hairs, macro-and microhairs, and silica bodies are
taxonomically informative in grasses (Metcalfe 1960; Ellis 1979).
Watson and Dallwitz (1992) reported detailed description of the
leaf epidermis in numerous taxa, pointing out the significance of these
characters in the systematics of the Poaceae.
The leaf blade epidermis in the Poaceae is divided into the costal
and intercostal zones (Metcalfe, 1960). The zones are more obvious in
species that have well-developed sclerenchyma strands associated with
vascular bundles just below the epidermis (Ellis,1979). The leaf
epidermis is composed of long cells and short cells, named for their
degree of elongation (Metcalfe, 1960 & Ellis, 1979).
Generally, long cells comprise more surface area of the leaf blade
(Ellis, 1979). Cell walls of long cells vary in their degree of undulation
and pitting (Metaclfe, 1960). Anticlinal walls of long cells may be
parallel, forming rectangular cells, angled outward to form hexagonal
cells, or bowed outward to form inflated cells. Long cells show a high
degree of phenotypic and developmental variation, thus taxonomic
significance of these characters must be inferrred with caution (Ellis,
1979). Short cells occur in rows in the coastal region, and in pairs or
individually between long cells in the intercostals region. Part (1948)
described short cells as "differentiated elements", and described four
categories: 1. silica cells, 2. exodermic elements ( i.e. hairs and prickles),
3.cork cells, and 4.stomata.
The comparative studies of epidermal characters using scanning
electron microscopy (SEM) to investigate East African grasses (Palmer
and Tucker,198,198; Palmer et al., 1985; Palmer and Gerbeth – Jones
1986,1988; Gomes, and Neves, 2009; Oratúñes and Fuenta, 2010) have
proven the taxonomic and phylogenetic utility of this technique.
SEM is revealed distinguished characters at the subfamily level
in the Poaceae, such as the fact that broad – tipped epidermal microhairs
are restricted to the Chloridoideae (Takeota et al.,1959; Amarasinghe and
Watson, 1990), while microhairs are absent in the Pooideae, except in
Lygeum Loefl. Ex L. and Nardus L.(GPWG 2001). Similarly, at the
generic level, morphology of epidermal papillae delineates species groups
within Sorghastrum Nash (Davila and Clark, 1990).
At the species level, Hilu (1984) revealed distinguishing epidermal
patterns in most Andropogon L. sect. Leptopogon, and phylogentically
informative characters have been reported in the epidermis of paleas and
lemmas in Bromus L. (Acedo and Liamas, 2001), Melica L.(Thomasson,
1986), and Zea mays L.(Part,1948), highlighting the utility of electron
microscopy in grass systematics .
Distribution and shape of silica bodies is taxonomically
informative at the tribal level in the Stipeae (Barkworth,1981). Certain
shapes of silica bodies are characteristic of grass subfamilies, e.g.
dumbbell- shaped in paicoid grasses, saddle-shaped in most pooid
grasses, and vertically oriented silica bodies in the Bambusoideae
(Piperno, 1988). Silica cells in the Poaceae are specialized cells in the
leaf epidermis that accumulate silica in a crystalline form, such that a
silica body, a type of phytolith, forms and occupies most of the lumen
(Piperno and Pearsall, 1998).
The significance and implications of silica bodies in taxonomy of
grasses have been widely addressed. Silica bodies are not restricted to
leaves, and an SEM survey of lemmas in structures is valuable for
differentiating species of Melica L. (Mejia-Saules and Bisby, 2003).
Silica bodies are preserved in the soil for up 600,000 years (Piperno and
Pearsall, 1998), and they were used to reconstruct changes in vegetation
patterns in Argentinean grasslands (Gallego and Distel, 2004).
In grasses the basic leaf-blade anatomy can be used to fined out if a
grass has C3 or C4 photosynthetic pathway. The two basic types of leaf–
blade anatomy are the Kranz and non-Kranz patterns of leaf anatomy
(Hattersley et al., 1977; Hatch, 1987; Chaudhary, 1989). The Kranz –type
possesses one or two bundle –sheaths per vascular bundle and 2-4
chlorenchymatous cells between adjacent vascular bundles compactly
radiating from or towards the bundle – sheaths. The starch formation here
takes place mainly in the bundle – sheath or in the parenchymatous
bundle – sheath (When the bundle – sheath is double with an inner sheath
of thick – walled cells). Non- Kranz type has two bundle sheaths per
vascular bundle. The chlorenchyma cells are irregularly diffused with
more than four cells between adjacent vascular bundles. The starch
formation here is mainly in the mesophyll. The above anatomical
structural types show a special significance when viewed from the
physiological and ecological points of view that the grasses with Kranztype of leaf – blade anatomy follow the C4 photosynthetic pathway and
the grasses with the non –Kranz type of leaf – blade follow the C3 or the
" normal " Calvin cycle of photosynthetic pathway. [In C3 photosynthetic
pathway a ribulose diphosphate molecule (5- carbon compound) picks up
diffused CO2 from mesophyll and gives rise to two phosphoglycerate
molecules ( a – 3 carbon compound and therefore the name C3 pathway )
. In C4 pathway, the plants have an extra cycle appended to the
photosynthetic
pathway.
Here
phosphor-enol-pyruvate
(3-carbon
compound ) takes up the diffused CO2 from the mesophyll and forms
aspartate or malate (4- carbon compounds and therefore the name C4
pathway: the latter product subsequently moves into the bundle sheath
where a decarbylating enzyme (NAD-ME or PEP-CK ) strips off the CO2
and passes It along for use in the C3cycle ] .
C4 pathway lends an advantage to the 'warm season' grasses in
warm temperature (and /or strong light) and arid climates through
increased photo-synthetic efficiency, higher water – use efficiency and,
relatively,greater drought tolerance. It was observed in Kenya (Tieszen et
al.,1979) that along an altitudinal and moisture gradient, nearly all the
high altitude grasses were of the C3 type and nearly all of the grasses of
low altitudes and low indices – of –available – moisture habitats were of
C4 types. The same is true to a great extent along a climate gradient but
yet within a hot region we may have both annual and perennial ' warm
climate ' grasses as well as mostly annual cool – season C3 grasses. Also,
there are C4 grasses which have adapted themselves to cool and / or
shady environment. Correlating with taxonomical groups, the sub –
families Arundinoideae, Bambusoideae and Poideae have the non –
Kranz types of leaf anatomy, whil the sub – family Panicoideae possesses
genera with Kranz and non – Kranz types of leaf anatomy (Clyton &
Renvoize, 1986). It is believed that the origin of C4 mode is polyphyletic.
2 .6 Classification of the family Poaceae
Subfamily: Panicoideae
Herbaceous grasses leaf - blades usually linear. Inflorescence a
panicle or of spikes (racemes) or compound. Spikelets singly arranged or
paired at nodes, 2-flowered, dorsally compressed, fanily entire, the florets
dimorphic and the rhachilla reduced to a callus, glumes or upper lemma
indurated, lodicules 2. Stamens 3, rarely fewer, stigmas 2, rarely 1. It
includes 7 tribes of which 3 are represented in Saudi Arabia.
Tribe: Paniceae
Inflorescence open or dense spicate panicle or unilateral spike
(racemes) arranged on an axis. Spikelets all alike, 2- flowered without
rachilla extensions (except in Brachiaria and Panicum), usually dorsally
compressed, fallining entire, rarely awaned, glumes abaxial in position
when the inflorescence racemose, usually membranous, rarely coriaceous
, the lower shorter and sometimes rudimentary, upper glume often as long
as the spikelet, membranous or herbaceous, rarely indurated, with or
without its corresponding palea, upper bisexual, the lemma and palea
indurated
.
Sub-tribe: Cenchrinae
Inflorescence of spicate or racemose panicles. Spikelets
dorsally compressed, borne singly or in clusters subtended by 1 or more
bristles or bracts (except Snowderia) often forming an involucre, all
falling off together, glumes shorter than the spikelet, often the lower
missing, lower lemma sterile, membranous about as long as the spikelet
rarely staminate and with palea, upper lemma cartilaginous to thinly
coriaceous with thin flat margins covering 1/2 to 2/3 of the palea.
It represents by 13 genera, four are represented in Saudi Arabia.
Pennisetum, Cenchrus, Snowdenia
and
Anthephora.