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The grass genera of the world

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

Arrhenatherum P. Beauv.

From the Greek arrhen, male, and ather, awn, alluding to the awn of the male floret.

Type species: Type: A. avenaceum P.Beauv., nom. illeg. = Avena elatior L. ??.

Excluding Pseudarrhenatherum

Habit, vegetative morphology. Perennial; caespitose. Culms 30–200 cm high; herbaceous; unbranched above; tuberous, or not tuberous. Culm nodes glabrous. Culm internodes hollow. Leaves not basally aggregated; non-auriculate. Leaf blades linear; apically flat; narrow; 2–7 mm wide; not setaceous; flat, or rolled (convolute); not pseudopetiolate; without cross venation; persistent; rolled in bud. Ligule an unfringed membrane (sometimes puberulent); not truncate; 0.5–3 mm long.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets, or without hermaphrodite florets (the upper floret being either hermaphrodite or female-only); outbreeding.

Inflorescence. Inflorescence paniculate; open. Primary inflorescence branches borne distichously. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets not secund; pedicellate.

Female-fertile spikelets. Spikelets 7–11 mm long; compressed laterally; disarticulating above the glumes (the florets falling together); not disarticulating between the florets (persistent). Rachilla prolonged beyond the uppermost female-fertile floret; hairy. Hairy callus present. Callus short.

Glumes two; very unequal; about equalling the spikelets to exceeding the spikelets; long relative to the adjacent lemmas; pointed; awnless; carinate, or non-carinate; similar (membranous). Lower glume 1 nerved, or 3 nerved. Upper glume 3(–5) nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets, or both distal and proximal to the female-fertile florets. The distal incomplete florets merely underdeveloped (when present). The proximal incomplete florets when present, 1 (occasionally, both or all florets perfect); paleate; male. The proximal lemmas awned (the awn geniculate, from the lower back); 5–9 nerved; more or less equalling the female-fertile lemmas; similar in texture to the female-fertile lemmas; not becoming indurated.

Female-fertile florets 1 (or rarely 2–4). Lemmas decidedly firmer than the glumes; not becoming indurated; entire, or incised; when incised not deeply cleft (notched); awnless, or awned. Awns when present, 1; dorsal; non-geniculate (usually short and slender); much shorter than the body of the lemma to about as long as the body of the lemma; entered by one vein. Lemmas hairy, or hairless; non-carinate; without a germination flap; 5–9 nerved. Palea present; relatively long; apically notched (shortly bidentate); 2-nerved; 2-keeled. Lodicules present; 2; free; membranous; glabrous; not toothed. Stamens 3. Anthers 3.4–6.5 mm long; not penicillate. Ovary apically hairy. Styles free to their bases. Stigmas 2; white.

Fruit, embryo and seedling. Fruit small to large; not grooved; compressed dorsiventrally to not noticeably compressed; hairy on the body. Hilum long-linear. 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 long mesocotyl; with a loose coleoptile, or with a tight coleoptile. First seedling leaf with a well-developed lamina. The lamina narrow; 3 veined.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells similar in shape costally and intercostally; of similar wall thickness costally and intercostally (fairly thin walled). Mid-intercostal long-cells rectangular (long and narrow); having straight or only gently undulating walls. Microhairs absent. Stomata common. Subsidiaries parallel-sided. Guard-cells overlapped by the interstomatals. Intercostal short-cells absent or very rare. Prickles abundant costally and intercostally. Costal short-cells neither distinctly grouped into long rows nor predominantly paired. Costal silica bodies horizontally-elongated crenate/sinuous, or horizontally-elongated smooth (mostly), or rounded.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with non-radiate chlorenchyma. Leaf blade ‘nodular’ in section, or adaxially flat; with the ribs very irregular in sizes. Midrib not readily distinguishable; with one bundle only. Bulliforms in simple fans (the cells fairly uniformly sized, or occasionally in irregular groups). All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; nowhere forming ‘figures’. Sclerenchyma all associated with vascular bundles.

Culm anatomy. Culm internode bundles in one or two rings.

Phytochemistry. Tissues of the culm bases with little or no starch. Fructosans predominantly long-chain. Leaves without flavonoid sulphates (1 species).

Cytology. Chromosome base number, x = 7. 2n = 14, 28, and 42. 2, 4, and 6 ploid. Chromosomes ‘large’. Nucleoli disappearing before metaphase.

Classification. Watson & Dallwitz (1994): Pooideae; Poodae; Aveneae. Soreng et al. (2015): Pooideae; Poodae; Poeae; Aveninae. 4 species.

Distribution, phytogeography, ecology. Europe, Mediterranean.

Commonly adventive. Mesophytic to xerophytic; species of open habitats. Dry grassland, edges of woods, disturbed ground.

Economic aspects. Significant weed species: A. elatius. Cultivated fodder: A. elatius.

Hybrids. Intergeneric hybrids with Avena.

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

References, etc. Leaf anatomical: Metcalfe 1960; this project.

Illustrations. • A. elatius: Gardner, 1952. • A. elatius, as A. avenaceum: P. De Beauvois (1812). • A. elatius: Gibbs Russell et al., 1990. • A. elatius (as Avena), general aspect: Eng. Bot. (1872). • Culm node (A. elatius). • Bulbous culm base of A. elatius. • Spikelet details (A. elatius). • Spikelet detail (A. elatius with G1 removed). • Flower of A. elatius. • A. elatius, abaxial epidermis of leaf blade: this project. • A. elatius, transverse section of leaf blade: this project


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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