Poa L.
From the Greek poa (grass, especially as fodder).
Including Arctopoa (Griseb.) Probat., Neuropoa Clayton, Oreopoa Grand., Paneion Lunell, Parodiochloa C.E. Hubb., Poagrostis Raf.
Excluding Bellardiochloa, Dasypoa, Poidium
Habit, vegetative morphology. Annual, or perennial; rhizomatous, or stoloniferous, or caespitose, or decumbent. Culms (1–)4–150 cm high; herbaceous; unbranched above; tuberous, or not tuberous. Culm internodes hollow. Young shoots extravaginal, or intravaginal. Leaves usually mostly basal; non-auriculate. Sheath margins joined, or free. 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); nearly always narrow; 0.2–12 mm wide (rarely wider); 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); once-folded in bud; an unfringed membrane, or a fringed membrane (rarely); truncate, or not truncate; (0.1–)0.5–6(–15) mm long. Contra-ligule absent.
Reproductive organization. Plants bisexual, with bisexual spikelets (nearly always, except in the Americas), or dioecious (e.g. in subgenus Andinae and assorted other species, 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 borne distichously (mostly), or inserted all around the main axis. 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, 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. Spikelets without proximal incomplete florets.
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); pointed; awnless, or mucronate (2–3 mm terminal ‘awns’ in a very few species, e.g. in the southern South American P. flabellata); 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; 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, or not toothed; not or scarcely vascularized. Stamens 3. Anthers 0.2–3 mm long; not penicillate; without an apically prolonged connective. Ovary glabrous; without a conspicuous apical appendage. Styles free to their bases. Stigmas 2; white.
Fruit, embryo and seedling. Fruit free from both lemma and palea; small; generally brown?; fusiform; longitudinally grooved (rarely?), or not grooved (usually); compressed laterally (occasionally?), or compressed dorsiventrally to not noticeably compressed; glabrous. Hilum short. 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; 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.
Taxonomy. Pooideae; Poodae; Poeae.
Distribution, ecology, phytogeography. About 500 species; cosmopolitan. Commonly adventive. Helophytic (rarely), or mesophytic (mostly), or xerophytic (rarely); shade species and species of open habitats; mostly glycophytic, or halophytic (a few). Typically in grasslands and meadows, a few (e.g. P. macrantha, P. confinis) in coastal sand.
Holarctic, Paleotropical, Neotropical, Australian, and Antarctic. Boreal, Tethyan, and Madrean. African, Madagascan, Indomalesian, Polynesian, and Neocaledonian. Arctic and Subarctic, Euro-Siberian, Eastern Asian, Atlantic North American, and Rocky Mountains. Macaronesian, Mediterranean, and Irano-Turanian. Saharo-Sindian, Sudano-Angolan, West African Rainforest, and Namib-Karoo. Indian, Indo-Chinese, Malesian, and Papuan. Hawaiian. Caribbean, Central Brazilian, Pampas, and Andean. North and East Australian and South-West Australian. New Zealand and Patagonian. European and Siberian. Canadian-Appalachian, Southern Atlantic North American, and Central Grasslands. Sahelo-Sudanian, Somalo-Ethiopian, and South Tropical African. Tropical North and East Australian and Temperate and South-Eastern Australian.
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, ‘Uromyces’ dactylidis, and Puccinia monoica. Smuts from Tilletiaceae and from Ustilaginaceae. Tilletiaceae — Entyloma, Tilletia, and Urocystis. Ustilaginaceae — Ustilago.
Economic importance. 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.
References, etc. Morphological/taxonomic: Vickery 1970. Leaf anatomical: mainly Metcalfe 1960; this project.
Illustrations. • General aspect. • General aspect. • Ligule. • Inflorescence and spikelet. • Inflorescence. • Inflorescence detail. • Inflorescence detail. • Inflorescence detail. • Inflorescence detail. • Spikelets. • Spikelet. • Spikelets. • Spikelet. Poa annua. • Abaxial epidermis of leaf blade. Poa annua. • Abaxial epidermis of leaf blade. Poa annua. • Transverse section of leaf blade. Poa sieberana. • Pollen antigens. • Pollen antigens. • 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
Cite this publication as: Watson, L., and Dallwitz, M. J. (1992 onwards). ‘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.’ http://biodiversity.uno.edu/delta/. Version: 18th August 1999. Dallwitz (1980), Dallwitz, Paine and Zurcher (1993 onwards, 1998), and Watson and Dallwitz (1994), and Watson, Dallwitz, and Johnston (1986) should also be cited (see References).
