The grass genera of the world

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L. Watson and M. J. Dallwitz

Avena L.

From the Latin avena (nourishment) and name for the oat, A. sativa.

Including Anelytrum Hack., Preissia Opiz

Habit, vegetative morphology. Annual; caespitose to decumbent. Culms 20–200 cm high; herbaceous; unbranched above. Culm nodes glabrous. Culm internodes hollow. Leaves not basally aggregated; non-auriculate. Leaf blades linear; narrow, or broad; 1.5–20 mm wide; flat (usually), or rolled (rarely convolute); without cross venation; persistent; rolled in bud, or once-folded in bud. Ligule present; an unfringed membrane; not truncate; 1–7 mm long.

Reproductive organization. Plants bisexual, with bisexual spikelets; with hermaphrodite florets; inbreeding; exposed-cleistogamous, or chasmogamous.

Inflorescence. Inflorescence paniculate; open; with capillary branchlets, or without capillary branchlets. Primary inflorescence branches borne distichously. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets not secund; pedicellate; not in distinct ‘long-and-short’ combinations.

Female-fertile spikelets. Spikelets 10–45 mm long; compressed laterally; disarticulating above the glumes, or not disarticulating (cultivated forms); when disarticulating above the glumes, not disarticulating between the florets, or disarticulating between the florets. Rachilla prolonged beyond the uppermost female-fertile floret; hairy, or hairless; the rachilla extension with incomplete florets. Hairy callus present, or absent. Callus pointed.

Glumes two (lanceolate); more or less equal; about equalling the spikelets, or exceeding the spikelets (rarely shorter); long relative to the adjacent lemmas; pointed; awnless; non-carinate; similar (usually chaffy). Lower glume 3–11 nerved. Upper glume 3–11 nerved. Spikelets with incomplete florets. The incomplete florets distal to the female-fertile florets. Spikelets without proximal incomplete florets.

Female-fertile florets (1–)2–6. Lemmas similar in texture to the glumes (rarely), or decidedly firmer than the glumes (usually leathery to crustaceous); becoming indurated to not becoming indurated; incised; 2 lobed (bidentate or 2-aristulate); not deeply cleft; awnless, or awned. Awns when present, 1, or 3; median, or median and lateral; the median different in form from the laterals (when laterals present); dorsal; from well down the back; geniculate; much longer than the body of the lemma; entered by one vein. The lateral awns shorter than the median. Lemmas hairy, or hairless; non-carinate; without a germination flap; 5–9 nerved. Palea present; relatively long, or conspicuous but relatively short, or very reduced (but large); entire to apically notched; tough; 2-nerved; 2-keeled. Lodicules present; 2; free; membranous; glabrous; toothed, or not toothed. Stamens 3. Anthers 0.7–4 mm long; not penicillate. Ovary hairy; without a conspicuous apical appendage. Styles free to their bases. Stigmas 2; white.

Fruit, embryo and seedling. Fruit adhering to lemma and/or palea, or free from both lemma and palea; medium sized; longitudinally grooved; compressed dorsiventrally; 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, or with a scutellar tail to without a scutellar tail; with a negligible mesocotyl internode. Embryonic leaf margins meeting, or meeting to overlapping.

Seedling with a long mesocotyl; with a loose coleoptile, or with a tight coleoptile. First seedling leaf with a well-developed lamina. The lamina broad, or narrow; erect; 7–15 veined.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells similar in shape costally and intercostally (long and narrow); of similar wall thickness costally and intercostally (fairly thin walled). Mid-intercostal long-cells fusiform (slightly); 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. 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 occasionally cubical).

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with non-radiate chlorenchyma; without adaxial palisade. Leaf blade with distinct, prominent adaxial ribs, or ‘nodular’ in section, or adaxially flat; with the ribs more or less constant in size. Midrib conspicuous, or not readily distinguishable; with one bundle only. Bulliforms present in discrete, regular adaxial groups, or not present in discrete, regular adaxial groups (the groups sometimes ill defined, or only hinge groups present); in simple fans (or in groups of fairly evenly sized cells). 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.

Cytology. Chromosome base number, x = 7. 2n = 14, 28, 42, 48, and 63. 2, 4, 6, 7, and 9 ploid. Chromosomes ‘large’. Haploid nuclear DNA content 4.3–5.5 pg (20 species, mean 4.7). Mean diploid 2c DNA value 9.9 pg (13 species, 8.8–11.0). Nucleoli disappearing before metaphase.

Taxonomy. Pooideae; Poodae; Aveneae.

Distribution, ecology, phytogeography. 27 species; Europe, Mediterranean, North Africa, western Asia. Commonly adventive. Mesophytic, or xerophytic; species of open habitats; glycophytic. Mostly in weedy places.

Holarctic, Paleotropical, Neotropical, and Cape. Boreal and Tethyan. African and Indomalesian. Euro-Siberian, Eastern Asian, and Atlantic North American. Macaronesian, Mediterranean, and Irano-Turanian. Saharo-Sindian and Sudano-Angolan. Indian and Indo-Chinese. Pampas. European and Siberian. Canadian-Appalachian, Southern Atlantic North American, and Central Grasslands. Somalo-Ethiopian.

Hybrids. Intergeneric hybrids with Arrhenatherum.

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. Ustilaginaceae — Ustilago.

Economic importance. Significant weed species: A. barbata, A. byzantina, A. fatua, A. ludoviciana, A. sterilis, A. strigosa, A. wiestii. Grain crop species: mainly A. sativa (Oats).

References, etc. Morphological/taxonomic: Baum 1977. Leaf anatomical: Metcalfe 1960; this project.

Illustrations. • General aspect (A. fatua). • Spikelet close-up (A. barbata). • Spikelet of A. fatua with glumes removed. • Spikelet of A. barbata without its glumes. • Lemma detail (A. barbata). • Lemma. • Caryopsis. • Caryopsis. • Starch from caryopsis. • Abaxial epidermis of leaf blade (Avena fatua)


This description is offered for casual browsing only. We strongly advise against extracting comparative information from it. This is much more easily achieved using 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 specified attributes, summaries of attributes within groups of taxa, geographical distribution, classification, and species sampled for anatomy.

Cite this publication as: ‘Watson, L., 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: 12th August 2014. http://delta-intkey.com’.

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