Poaceae Barnhart
  • Barnhart, Bull. Torrey Bot. Club 22: 7 (1895)


This taxon is accepted by WCS higher taxonomy

General Description

Root, culm & leaf


The life cycle of most grasses is strongly seasonal. Perennials become dormant when the season is adverse to growth, allowing some or all of their foliage to wither after recovering any useful nutrients contained in it; truly deciduous leaves are very rare. Annuals overwinter as seed, a strategy generally regarded as more advanced because of the high standard of reproductive efficiency required. Perennials may be recognized by the presence of dormant buds or innovation shoots at their base, but these are not always obvious in the weedy short lived perennials produced by some tropical genera, nor in rambling species that are difficult to trace back to their rootstock. The root system is fibrous and relatively shallow, seldom penetrating more than 2 m and often less than 1 m (Troughton 1957). Commonly about half of it dies and is replaced each year. The main branch system and the perennating buds occur just above (tussock grasses), at (stoloniferous and sward-forming grasses) or just below (rhizomatous grasses) the soil surface. It is relatively inaccessible to grazing, and readily produces new shoots ('tillers') from its axils to make good any loss of the aerial parts. Adventitious roots develop freely, so that typically each tiller is sustained by its own independent root system. The tillers eventually develop into upright inflorescence-bearing culms. The culms are usually unbranched, though there are some notable exceptions especially in Andropogoneae; they are mostly hollow, but may be solid particularly in Chloridoideae and Panicoideae (Brown et all959a). In a few species the base of the culm develops into a food-storing bulb or corm (Bums 1946). The usual Monocotyledonous division of the leaf into sheath and blade is highly developed in grasses, and gives rise to their characteristic life-form. The most obvious manifestation is the manner in which the stem and sheath combine to form a structural unit. As a result the apical primordium is protected within a nest of concentric sheaths, and the internode can elongate at a soft basal intercalary meristem which extrudes it from the supporting sheath. Intercalary meristems also feature in the node (or sheath base, Brown et al 1959b ), which retains the ability to turn the culm upright after lodging by rain or trampling; and in the blade, which grows from a basal meristem (the 'collar') and can thus recover from loss of its distal parts by cropping. The blade is typically linear with the ability to roll or fold under desiccation, but a wide range of other shapes occasionally occurs. It is the primary photosynthetic organ, but the sheath (Throne 1959) and even the awns (Grandbacher 1963) can make an important contribution. At the junction of sheath and blade stands the ligule, represented by a membrane or by a line of hairs. Its function is obscure but may plausibly be to deter insects from entering the sheath; it is a novel structure, not homologous with any other organ (Philipson 1935). Very rarely there is a ligule-like structure on the abaxial side of the sheath (Tran 1971 ), or amides at base of blade or summit of sheath (Tran 1972). Other variations are an abscission zone at the collar, or a basal narrowing of the blade so extreme that it imitates a petiole. Note that the first leaf of an axillary branch is adaxial, highly modified and termed a prophyU. It is membranous and 2-keeled, scale-like on basal branches but linear on aerial branches. It is often difficult to find and seems to be of slight practical importance, but it is of interest in establishing homologies. The ligule is often a useful aid to tribal recognition, though prone to erratic exceptions. Otherwise vegetative characters are seldom of much taxonomic significance above generic level.


Inflorescence


The inflorescence is a special branch system whose subtending leaves have been suppressed. It may be a panicle, open or contracted; a spiciform panicle, where contraction has proceeded to the point of fusing most of the branches to the central axis; a single raceme with spikelets on one side of the rhachis, on opposite sides, or all around like a bottle-brush; or multiple racemes arranged digitately or scattered along an axis.Terminology is sometimes rather strained, as when a panicle is so sparse that its branches resemble racemes, when a raceme bears little spikelet dusters instead of single spikelets, or when its spikelets have no pedicel. The latter is sometimes distinguished as a spike, but the difference is often so slight, or downright contradictory in Andropogoneae, that this refinement is more confusing than helpful. The terminology in any case is technically incorrect as it is derived by treating its basic unit, the spikelet, as analogous to the normal petaloid flower. However, there seems little point in replacing a well established convention with a plethora of new terms. The inflorescence is often quite characteristic of a tribe, and is so obvious that it can scarcely be ignored. However, it should be treated with caution, as it is also the character most likely to mislead.


Spikelet


The spikelet is composed of bracts borne distichously on a rhachilla. The two lowest bracts ('glumes') are empty scales which protect the immature spikelet; the rest ('lemmas') are each opposed by a palea. Lemma and palea together enclose a set of floral organs, the whole forming a structural unit known, by functional analogy rather than morphological homology, as a floret. The base of the spikelet or floret is sometimes enlarged into a little knob or stalk called a callus. An enormous range of spikelet variation has been achieved by reducing the number of scales, elaborating their shape, or extending them into bristles called awns; yet the overall structural pattern is remarkably consistent, and there is seldom any difficulty in relating its variants to the standard form. Note that the suppression of a scale, which happens in a few genera, can be inferred from its interruption of the distichous sequence. The floral organs typically consist of2 tiny lodicules, 3 stamens and 2 plumose stigmas. Morphologically they are rather unifonn, but there is some variation in their number (Clifford 1961). This is often associated with varying degrees of sexual segregation, in which the most significant comparison is female or bisexual against male or barren. It is a convenient, though inexact, abbreviation to unite the two ovary-bearing forms under the term 'fertile'. It has been shown that glumes and lemmas are mainly derived from leaf sheaths, though their upper part may involve more complex homologies with ligule, aurides and blade (Tran 1 973). The pale a is commonly a hyaline 2-keeled structure, best regarded as the prophyll of a diminished axillary branch. The lodicules are usually interpreted as a vestigial perianth, which swells to open the floret for anther emergence, and then allows it to spring shut until the stigmas are ready for extrusion (Pissarek 1971). They are commonly lacking in cleistogamous and protogynous florets. It may be postulated that the spikelet is derived by condensation from a spike with axillary petaloid flowers; aggregation of the spikelets coupled with suppression of the intervening leaves then formed the grass panicle. Nor is that the end of it, for second order reduction-aggregation cycles may be observed in Bambuseae and Andropogoneae. The multiplication of floral envelopes is a notable feature of the grass life form. Thus the function of the lemma is often duplicated by an enlarged glume, and sometimes by sterile outer florets; in the most complex examples it may be further reinforced by an involucre of sterile spikelets or sterile branchlets, or by a modification of the uppermost leaf-sheath. This trend is evidently of importance to the plant, but its purpose is largely a matter for speculation. The most obvious effect is to increase the protection of flower and fruit. Also, since the 'sterile' florets are in fact often male, it increases the ratio of stamens to ovaries, Furthermore it has been shown that a large proportion of the seed's food store is photosvnthesized in the flag leaf and inflorescence itself (Redman & Reekie in Estes et al 1982), so that the additional envelopes may be a means of enhancing this process without obstructing the exposure of the inflorescence to anemophilous pollination. Characters drawn from the spikelet are fundamental to the taxonomy of grasses is invariably hazardous without careful dissection to observe their structure.


Cleistogamy


Cleistogamous spikelets in which self-fertilization occurs within the closed floret, are quite common, being reported for over 300 species. These spikelets, which are additional to the normal form, show varying degrees of modification Campbell et al, 1983).



  1. Spikelet fertilization: Inflorescence exposed, but florets fail to open. Rare, or at least difficult to detect since spikelet modification is minimal.


  2. Sheath fertilization: Infloresence or spikelets remain within upper sheaths during fertilization, but may be exserted later; spikelets usually smaller than normal. The commonest form.


  3. Cleistogenes: Modified spikelets within lowermost sheaths, sometimes associated with specialised tumbleweed dispersal involving parts of the culm and subtending leaves.


  4. Rhizanthogenes: Highly modified spikelets on specialized underground rhizomes.

Fruit


The grass fruit normally has a thin pericarp firmly adherent to the seed, and is by no means rare to find the pericarp free, the fruit then being a utricle if the pericarp is soft, or an achene if it is hard. The attachment of the ovule to the pericarp wall leaves a visible scar ('hilum') on the adaxial side which is of some taxonomic value. The bulk of the seed is made up of endosperm, that curious triploid tissue common to Angiosperms which is formed by fusion of two polar nuclei from the ovule with a secondary nucleus from the pollen. It contains starch and protein which provide the initial food supply for the seedling. Investigation of its sugar and amino acid chemistry has shown broad taxonomic correlations, but provides little diagnostic significance. The embryo has a flat haustorial cotyledon ('scutellum'), and a special outer sheath ('coleoptile') protecting the plumule during soil penetration (Negbi 1984). Numerous attempts have been made to establish precise homologies with foliage leaves (Brown 1960, Guignard & Mestre 1 970), but it seems just as likely that these are novel organs evolved to meet the special needs of germination. For taxonomic purposes the embryo structure may be expressed in a simple formula; its terms, and the principal combinations, are set out below. Other combinations also occur, the epiblast being particularly prone to erratic variation, but the microscope preparations are not always easy to interpret and this may account for some of the apparent anomalies. The most important sources are Reeder 1960 & 1962, Kinges 1961, Decker 1962 and Mlada 1977.


1. Mesocotyl present (P), absent (F)

2. Epiblast present(+), absent(-)

3. Scutellum cleft present (P), absent (F)

4. First leaf rolled (P), folded (F)


Principal combinations:

Bambusoid: F + PP

Oryzoid: F + FP

Centothecoid: P + PP

Pooid: F + FF

Arundinoid: P - PF

Chloridoid: P + PF

Panicoid: P - PP


The dispersal unit is rarely a seed, sometimes a caryopsis, more often a false fruit incorporating various parts of the spikelet or inflorescence. The false fruits are exceedingly  diverse, and are often equipped with elaborate appendages. Awns that propel the fruit along the ground by hygroscopic flexing or coiling are particularly common, backward movement being prevented by a beard on the callus. The awn is often complex with a straight limb set geniculately upon a twisted column; and sometimes the moving awn is designed to catch on a fixed awn until the mounting tension springs it free, making the fruit hop. Such devices are loosely referred to as dispersal mechanisms, but in fact many of them seem more concerned with establishment of the germinating seed. It is very difficult to secure experimental evidence relating form to function, but Peart (1984) has demonstrated the role of such devices in moving the fruit to a favourable microsite, orienting it to facilitate initial water uptake, and anchoring it securely against the thrust of the radicle. When the fruit incorporates a hard

lemma there is usually a neatly dehiscing germination flap at its base through which the radicle emerges (Johnston & Watson 1983). The fruits of many species survive burial in the ground for up 10 years; few exceed 30 years (Too le & Brown 1946).

Reproduction

Reproduction


Grasses have developed a wide range of breeding behaviour (Connor 1980, 1981), broadly divisible into two opposing strategies. Some have countered the incestuous promiscuity of anemophily by developing a complex incompatibility system that ensures outbreeding (Heslop-Harrison 1982); or, less often, by adopting dioecy. Others, particularly annuals, have reduced the uncertainty of anemophily by self-fertility or deistogamy. The more extreme forms of inbreeding are invariably facultative and often mediated by environmental conditions, thus mitigating their restrictive effect on genetic diversity. Cytogenetic systems are likewise extremely varied, with extensive development of polyploidy (Carnahan & Hill 1961, Stebbins in Younger & McKell 1972), hybridization (Knobloch 1968) and facultative apomixis (Nygren 1954, Brown 1958, Brown & Emery 1958); polyhaploidy, the reversion of polyploids to the diploid condition, has been reported in Bromus, Dactylis and Sorghum (Kimber & Riley 1963). Taken together these processes have produced systems of great flexibility, capable of responding conservatively or adaptively according to the exigencies of selection pressure. Their ability to proliferate segregate populations, which yet retain some capacity for gene exchange, has often created polymorphic complexes of fearsome taxonomic difficulty, but their versatility has been a potent factor in the success of the grasses. The chromosomes themselves are of limited taxonomic interest, as the karyotype is relatively constant and the chromosome count, though variable, shows only broad statistical correlation with the classification. Their evolutionary history is largely speculative, but it seems likely that 12 was the primitive basic number, with 10 and 9 derived from it. The most significant deviation occurs in the Pooideae, where large chromosomes with a basic number of 7 are associated with the more advanced tribes.


Breeding behaviour


Grasses have developed a wide range of breeding behaviour (Connor 1980, 1981), broadly divisible into two opposing strategies. Some have countered the incestuous promiscuity of anemophily by developing a complex incompatibility system that ensures outbreeding (Heslop-Harrison 1982); or, less often, by adopting dioecy. Others, particularly annuals, have reduced the uncertainty of anemophily by self-fertility or cleistogamy. The more extreme forms of inbreeding are invariably facultative and often mediated by environmental conditions, thus mitigating their restrictive effect on genetic diversity. Cytogenetic systems are likewise extremely varied, with extensive development of polyploidy (Carnahan & Hill 1961, Stebbins in Younger & McKell 1972), hybridization (Knobloch 1968) and facultative apomixis (Nygren 1954, Brown 1958, Brown & Emery 1958); polyhaploidy, the reversion of polyploids to the diploid condition, has been reported in Bromus, Dactylis and Sorghum (Kimber & Riley 1963). Taken together these processes have produced systems of great flexibility, capable of responding conservatively or adaptively according to the exigencies of selection pressure. Their ability to proliferate segregate populations, which yet retain some capacity for gene exchange, has often created polymorphic complexes of fearsome taxonomic difficulty, but their versatility has been a potent factor in the success of the grasses. The chromosomes themselves are of limited taxonomic interest, as the karyotype is relatively constant and the chromosome count, though variable, shows only broad statistical correlation with the classification. Their evolutionary history is largely speculative, but it seems likely that 12 was the primitive basic number, with 10 and 9 derived from it. The most significant deviation occurs in the Pooideae, where large chromosomes with a basic number of7 are associated with the more advanced tribes.



Vivipary and proliferation


True vivipary, the precocious germination of the seed while still attached to the parent, is unusual. However, the term is commonly applied to a few species, mainly Arctic-alpine Pooideae, that regularly propagate by means of plantlets formed by vegetative proliferation of the spikelets (Langer & Ryle 1958, Beetle 1980). A teratological version of proliferation is not uncommon, particularly in the autumn when it seems to be induced by the effect of cold and short days on genetically predisposed races. Hormone weedkillers and pathological invasions can have a similar effect. 


Vegetative reproduction.


Seed set in perennials is often surprisingly low, and vegetative reproduction is evidently of considerable importance as an alternative strategy, its effectiveness being greatly enhanced by the evolution of stolons and rhizomes as a means of dispersal. The size and age of single clones is difficult to investigate but Harberd (1961, 1962), using self-incompatibility as an indicator of clonal membership, has estimated 200 metres and 400 years for Festuca rubra, 8 meters and 1000 years for F. ovina.

Included Genus

  Bibliography

  • 1 Clayton, D.W. & Renvoize, S.A. Genera graminum: grasses of the World. Kew Bulletin Additional Series XIII (1986).
  • 2 Barnhart, J.H. Original publication of Poaceae. (1895).

 Information From

WCS higher taxonomy
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