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The grass genera of the world

L. Watson, T.D. Macfarlane, and M.J. Dallwitz

Poa L.

From the Greek poa (grass, especially as fodder).

Type species: Type: P. pratensis L.

Including Arctopoa (Griseb.) Probat., Neuropoa Clayton, Oreopoa Grand., Paneion Lunell, Parodiochloa C.E. Hubb., Poagrostis Raf.

Excluding Austrofestuca, Bellardiochloa, Dasypoa, Nicoraepoa, Poidium, etc.

Habit, vegetative morphology. Annual, or perennial; rhizomatous, or stoloniferous, or caespitose, or decumbent. Culms (1–)4–150 cm high; herbaceous; branched above, or unbranched above (usually). The branching simple. Culms tuberous, or not tuberous. Culm internodes hollow. Young shoots extravaginal, or intravaginal. Leaves usually mostly basal; with vestigial auricles, or non-auriculate. Sheath margins joined (usually, at least basally). Leaf blades linear, or linear-lanceolate (often ending in a boat-shaped tip, and the adaxial surface characteristically with a longitudinal groove on either side of the midrib); apically flat (P. hookeri), or apically cucullate (mostly); nearly always narrow; 0.2–12 mm wide (rarely wider); only rarely apically pungent, setaceous, or not setaceous; flat, or folded (or canaliculate), or rolled (involute or convolute); without cross venation; persistent (usually), or disarticulating from the sheaths (very rarely); rolled in bud (rarely), or once-folded in bud. Ligule an unfringed membrane, or a fringed membrane (rarely); truncate, or not truncate; (0.1–)0.5–6(–18) mm long. Contra-ligule absent.

Reproductive organization. Plants bisexual, all with bisexual spikelets (nearly always, except in the Americas), or dioecious (in assorted species even with Nicoraepoa excluded, and gynodioecious in about 50 species constituting Poa sect. Dioicopoa); with hermaphrodite florets, or without hermaphrodite florets (rarely). The spikelets hermaphrodite, or female-only, or hermaphrodite and female-only, or male-only. Plants outbreeding and inbreeding; exposed-cleistogamous, or chasmogamous. Apomictic (pseudogamous and non-pseudogamous), or reproducing sexually. Viviparous (sometimes), or not viviparous.

Inflorescence. Inflorescence reduced to a single spikelet to few spikeleted (very rarely), or many spikeleted (nearly always); paniculate; open, or contracted; with capillary branchlets, or without capillary branchlets. Primary inflorescence branches nearly always borne distichously. Rachides subterete or flattened, or winged, or neither flattened nor hollowed, not winged (?). Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets not secund; pedicellate.

Female-sterile spikelets. The male spikelets with glumes; without proximal incomplete florets.

Female-fertile spikelets. Spikelets 2–11 mm long; compressed laterally; disarticulating above the glumes; disarticulating between the florets. Rachilla prolonged beyond the uppermost female-fertile floret; hairless (nearly always glabrous). Hairy callus present (mostly dorsally webbed or with a crown of hairs), or absent. Callus short; blunt.

Glumes two; more or less equal, or very unequal to more or less equal (nearly always ‘subequal’, with the G1 somewhat shorter); shorter than the spikelets; shorter than the adjacent lemmas; free; pointed, or not pointed (rarely rounded); awnless; carinate; similar (membranous). Lower glume 1 nerved, or 3 nerved. Upper glume not saccate; (1–)3(–5) nerved. Spikelets with female-fertile florets only, or with incomplete florets. The incomplete florets distal to the female-fertile florets. The distal incomplete florets merely underdeveloped.

Female-fertile florets (1–)2–13(–15) (the one-floreted species very unusual, perhaps restricted to Malesia). Lemmas similar in texture to the glumes; not becoming indurated; nearly always entire (very rarely tridenticulate); typically pointed; nearly always awnless (with Parodiochloa excluded?); hairy (often with web-like hairs), or hairless (rarely, but sometimes glabrous in male florets of dioecious species); carinate; without a germination flap; (3–)5 nerved, or 7–11 nerved (rarely, e.g. in the Australian Neuropoa); with the nerves non-confluent. Palea present; relatively long, or conspicuous but relatively short; usually tightly clasped by the lemma; entire to apically notched (emarginate); awnless, without apical setae; thinner than the lemma, or textured like the lemma; 2-nerved; 2-keeled. Palea keels wingless; glabrous to hairy (glabrous, aculeolate or ciliate). Lodicules present; 2; free; membranous; nearly always glabrous (occasionally ciliolate); toothed (usually asymmetrically bilobed), or not toothed; not or scarcely vascularized. Stamens 3. Anthers 0.2–3 mm long; not penicillate; without an apically prolonged connective. Ovary apically glabrous. Styles free to their bases. Stigmas 2; white.

Fruit, embryo and seedling. Fruit free from both lemma and palea; small (1–4 mm long); generally brown?; fusiform, or ellipsoid; at least somewhat longitudinally grooved, or not grooved; compressed laterally (occasionally?), or compressed dorsiventrally to not noticeably compressed; glabrous. Hilum short (0.2–0.4 mm). Embryo small; not waisted. Endosperm hard; with lipid; containing compound starch grains. Embryo with an epiblast; without a scutellar tail; with a negligible mesocotyl internode. Embryonic leaf margins meeting.

Seedling with a short mesocotyl, or with a long mesocotyl; with a loose coleoptile, or with a tight coleoptile. First seedling leaf with a well-developed lamina. The lamina narrow; erect; 3 veined, or 5 veined.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous (usually), or lacking. Papillae absent (usually), or present (e.g. P. sieberana, P. helmsii); when present, intercostal, or costal and intercostal. Intercostal papillae not over-arching the stomata; several per cell (of the coronate-pit type, illustrated in Clifford and Watson 1977). Long-cells similar in shape costally and intercostally; of similar wall thickness costally and intercostally (thick or thin walled). Mid-intercostal long-cells rectangular, or fusiform; having markedly sinuous walls, or having straight or only gently undulating walls. Microhairs absent. Stomata absent or very rare, or common; when present, (27–)30–54(–57) microns long. Subsidiaries non-papillate; parallel-sided, or dome-shaped (low), or parallel-sided and dome-shaped. Guard-cells overlapped by the interstomatals. Intercostal short-cells common, or absent or very rare; in cork/silica-cell pairs, or not paired; silicified, or not silicified. Intercostal silica bodies when present, tall-and-narrow, or crescentic. Costal short-cells predominantly paired, or neither distinctly grouped into long rows nor predominantly paired. Costal silica bodies horizontally-elongated crenate/sinuous, or horizontally-elongated smooth, or rounded, or tall-and-narrow, or crescentic; not sharp-pointed.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with non-radiate chlorenchyma; without adaxial palisade. Leaf blade ‘nodular’ in section, or adaxially flat; with the ribs more or less constant in size. Midrib conspicuous (usually groved on either side); with one bundle only. Bulliforms present in discrete, regular adaxial groups, or not present in discrete, regular adaxial groups (commonly restricted to the ‘midrib hinge’ pair); in simple fans. Many of the smallest vascular bundles unaccompanied by sclerenchyma, or all the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present, or absent; forming ‘figures’, or nowhere forming ‘figures’. Sclerenchyma all associated with vascular bundles, or not all bundle-associated. The ‘extra’ sclerenchyma when present, in abaxial groups, or in a continuous abaxial layer (e.g. P. labillardieri).

Phytochemistry. Tissues of the culm bases with little or no starch. Fructosans predominantly long-chain.

Cytology. Chromosome base number, x = 7. 2n = 14, or 28, or 35, or 38, or 38–117, or 42, or 43, or 44, or 56, or 50–56, or 63, or 65, or 70–72, or 76 (etc.). 2, 4, 5, 6, 7, 8, 9, 10, 11, and 12 ploid (etc., and aneuploids). Chromosomes ‘large’. Haploid nuclear DNA content 0.9–2.8 pg (6 species, mean 1.5). Mean diploid 2c DNA value 3.6 pg (3 species, 2.4–5.6). Nucleoli disappearing before metaphase.

Classification. Watson & Dallwitz (1994): Pooideae; Poodae; Poeae. Soreng et al. (2015): Pooideae; Poodae; Poeae; Poinae. About 500 species.

Distribution, phytogeography, ecology. Cosmopolitan.

Tasmania, New South Wales, Australian Capital Territory, Victoria, Western Australia, Queensland, South Australia, New Guinea, and New Zealand.

Commonly adventive. Helophytic (rarely), or mesophytic (mostly), or xerophytic (rarely); shade species and species of open habitats; mostly glycophytic. Typically in grasslands and meadows, a few (e.g. P. macrantha, P. confinis) in coastal sand.

Economic aspects. Significant weed species: P. annua, P. bulbosa, P. compressa, P. infirma, P. pratensis, P. sphondylodes, P. sylvicola, P. trivialis. Cultivated fodder: notably P. palustris, P. pratensis (Meadow, Bluegrass), P. compressa. Important native pasture species: many valuable: e.g. P. alpina, P. arctica, P. arida, P. compressa, P. epilis, P. interior, P. gracillima, P. juncifolia, P. palustris, P. pratensis, P. rupicola, P. schimperana, P. trivialis. Lawns and/or playing fields: P. nemoralis, P. pratensis, P. trivialis etc. - the generally inoffensive P. annua being commonly present as a weed.

Hybrids. Intergeneric hybrid of Arctopoa (= Poa) with Dupontia: ×Dupontopoa N.S. Probatova (exemplified by Poa labradorica Steudel, with ×Dupontopoa dezhnevii representing taxonomic errors: see Darbyshire et al. 1992, Darbyshire and Cayouette 1992).

Rusts and smuts. Rusts — Puccinia. Taxonomically wide-ranging species: Puccinia graminis, Puccinia coronata, Puccinia striiformis, Puccinia brachypodii, Puccinia recondita, ‘Uromycesdactylidis, and Puccinia monoica. Smuts from Tilletiaceae and from Ustilaginaceae. Tilletiaceae — Entyloma, Tilletia, and Urocystis. Ustilaginaceae — Ustilago.

References, etc. Morphological/taxonomic: Vickery 1970. Leaf anatomical: Metcalfe 1960; this project.

Illustrations. • P. alpina, general aspect: Eng. Bot. (1872). • P. annua, general aspect: Eng. Bot. (1872). • P. annua, general aspect: Gibbs Russell et al., 1990. • P. bulbosa, general aspect: Eng. Bot. (1872). • P. compressa, general aspect: Eng. Bot. (1872). • cf. P. flexuosa (as P. laxa), general aspect: Eng. Bot. (1872). • P. glauca (as P. caesia), general aspect: Eng. Bot. (1872). • P. glauca (as P. balfourii), general aspect: Eng. Bot. (1872). • P. glauca (as P. dissitiflora), general aspect: Eng. Bot. (1872). • cf. P. lindebergii (as P. stricta), general spect: Eng. Bot. (1872). • P. nemoralis, general aspect: Eng. Bot (1872). • cf. P. nemoralis (as var. parnelii), general aspect: Eng. Bot. (1872). • P. pratensis, general aspect: Eng. Bot. (1872). • P. pratensis: Gardner, 1952. • P. trivialis, general aspect: Eng. Bot. (1872). • P. minimiflora: Hook. Ic. Pl. 27 (1901). • P. papuana: Hook. Ic. Pl. 27 (1901). • Ciliolate ligule of P. morrisii. • Inflorescence and spikelet of P. drummondiana. • a typical inflorescence. • Inflorescence detail (P. affinis). • Inflorescence detail (P. affinis). • Inflorescence detail (P. affinis). • Inflorescence detail (P. morrisii). • Spikelets of P. morrisii. • Spikelet of P. costiniana. • typical spikelets. • Spikelet of P. annua. • P. foliosa (as Festuca): Hooker, Fl. Antarctica (1844). • P. cookii (as Festuca): Hooker, Fl. Antarctica (1844). • P. alopecurus (as Festuca fuegiana): Hooker, Fl. Antarctica (1844). • P. flabellata (as Dactylis): Hooker, Fl. Antarctica (1844). • P. tenera and P. (Saxipoa) saxicola: Hooker, Fl. Tasmaniae (1860). • P. annua, abaxial epidermis of leaf blade: this project. • P. annua, abaxial epidermis of leaf blade: this project. • P. helmsii, abaxial epidermis of leaf blade with coronate pits: original. Coronate pits in high focus. • P. helmsii, abaxial epidermis of leaf blade with coronate pits: original. Coronate pits in low focus. • P. sieberana, T.S. leaf blade - fluorescence image: this project. • Pollen antigens: Watson and Knox (1976). • Pollen antigens: cross-reactions against anti-Lolium serum. • Pollen antigens: cross-reactions against anti-Lolium serum. • Heat stable pollen antigens (allergens): cross-reactions against anti-Lolium serum. • Pollen antigens: cross-reactions against anti-Cynodon serum. • Pollen antigens: cross-reactions against anti-Zea serum. • Pollen antigens: cross-reactions against anti-Zea serum


We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, and classifications. See also Guidelines for using data taken from Web publications.


Cite this publication as: ‘Watson, L., Macfarlane, T.D., and Dallwitz, M.J. 1992 onwards. The grass genera of the world: descriptions, illustrations, identification, and information retrieval; including synonyms, morphology, anatomy, physiology, phytochemistry, cytology, classification, pathogens, world and local distribution, and references. Version: 13th November 2017. delta-intkey.com/grass’.

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