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The grass genera of the world

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

Aira L.

The Greek name for Lolium temulentum.

Hairgrasses.

Type species: Type: A.caerulea L.

Including Airella (Dumort.) Dumort., Aspris Adans., Caryophyllea Opiz, Fiorinia Parl., Fussia Schur, Salmasia Bub.

Habit, vegetative morphology. Small, slender annual; caespitose. Culms 2–40 cm high; herbaceous; unbranched above. Culm nodes glabrous. Culm internodes hollow. Leaves mostly basal, or not basally aggregated; non-auriculate. Leaf blades linear; apically cucullate; narrow; 0.3–2 mm wide; setaceous; flat, or folded, or rolled; without abaxial multicellular glands; without cross venation; persistent; rolled in bud, or once-folded in bud. Ligule an unfringed membrane; not truncate (acute); 2.4–5 mm long.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets; inbreeding.

Inflorescence. Inflorescence paniculate; open, or contracted; when contracted, spicate to more or less irregular; with conspicuously divaricate branchlets, or without conspicuously divaricate branchlets; with capillary branchlets. 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 1.6–3.5 mm long; compressed laterally; disarticulating above the glumes; disarticulating between the florets; with conventional internode spacings. The upper floret not stipitate. Rachilla prolonged beyond the uppermost female-fertile floret, or terminated by a female-fertile floret; hairless; the rachilla extension when present, naked. Hairy callus present, or absent. Callus short; blunt.

Glumes two; more or less equal; about equalling the spikelets, or exceeding the spikelets; long relative to the adjacent lemmas; pointed; awnless; carinate, or non-carinate; similar (membranous, delicate). Lower glume 1–3 nerved. Upper glume 1–3 nerved. Spikelets with female-fertile florets only; without proximal incomplete florets.

Female-fertile florets 2. Lemmas decidedly firmer than the glumes (becoming papery); not becoming indurated; entire, or incised; awnless, or awned. Awns when present, 1; dorsal; from well down the back; geniculate; much shorter than the body of the lemma to much longer than the body of the lemma; entered by one vein. Lemmas hairless; carinate to non-carinate; 5 nerved. Palea present; relatively long; tightly clasped by the lemma; apically notched; awnless, without apical setae; 2-nerved. Lodicules present; 2; free; membranous; glabrous; not toothed. Stamens 3. Anthers 0.2–1.7 mm long; not penicillate. Ovary apically glabrous. Styles free to their bases. Stigmas 2; white.

Fruit, embryo and seedling. Fruit adhering to lemma and/or palea; small; fusiform; longitudinally grooved; compressed dorsiventrally. 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 long mesocotyl; with a loose coleoptile, or with a tight coleoptile. First seedling leaf with a well-developed lamina. The lamina narrow; erect; 3–5 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. Mid-intercostal long-cells fusiform; having markedly sinuous walls, or having straight or only gently undulating walls. Microhairs absent. Stomata absent or very rare, or common; 45–54 microns long (in A. cupaniana). Subsidiaries parallel-sided, or dome-shaped. Guard-cells overlapped by the interstomatals. Intercostal short-cells absent or very rare. Intercostal silica bodies absent. Costal short-cells conspicuously in long rows, or neither distinctly grouped into long rows nor predominantly paired. Costal silica bodies confined to the central file(s) of the costal zones; horizontally-elongated crenate/sinuous.

Transverse section of leaf blade, physiology. Lamina mid-zone in transverse section open.

C3; XyMS+. Mesophyll with non-radiate chlorenchyma; without adaxial palisade. Leaf blade with distinct, prominent adaxial ribs, or ‘nodular’ in section; with the ribs more or less constant in size. Midrib conspicuous; with one bundle only. Bulliforms not present in discrete, regular adaxial groups. Many of the smallest vascular bundles unaccompanied by sclerenchyma, or all the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders absent. Sclerenchyma all associated with vascular bundles.

Phytochemistry. Tissues of the culm bases with abundant starch, or with little or no starch. Leaves without flavonoid sulphates (1 species).

Cytology. Chromosome base number, x = 7. 2n = 14 and 28. 2 and 4 ploid. Chromosomes ‘large’. Haploid nuclear DNA content 2.7–3.2 pg (4 species, mean 2.9). Mean diploid 2c DNA value 6 pg, or 5.9 pg (A. elegantissima, A. praecox).

Classification. Watson & Dallwitz (1994): Pooideae; Poodae; Aveneae. Soreng et al. (2015): Pooideae; Poodae; Poeae; Airinae. 8 species.

Distribution, phytogeography, ecology. North and South temperate.

Commonly adventive. Mesophytic to xerophytic; species of open habitats. Sandy soils.

Rusts and smuts. Rusts — Puccinia. Taxonomically wide-ranging species: Puccinia graminis and Puccinia striiformis. Smuts from Tilletiaceae. Tilletiaceae — Tilletia.

References, etc. Leaf anatomical: this project.

Illustrations. • A. caryophyllea, general aspect: Eng. Bot. (1872). • A. praecox, general aspect: Eng. Bot. (1872). • A. cupaniana, general aspect: Gibbs Russell et al., 1990. • Inflorescence detail (A. elegantissima). • A. cupiana, abaxial epidermis of leaf blade: this project. • A. cupiana, 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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