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The grass genera of the world

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

Helictotrichon Besser ex Roem. & Schult.

From the Greek helictos (twisted) and "trichon", presumably referring to the awn.

Including Avenastrum Opiz, Avenochloa Holub, Avenula (Dumort.) Dumort., Danthorhiza Ten., Heuffelia Schur, Stipavena Vierh., Tricholemma

Excluding Amphibromus, Pseudarrhenatherum

Habit, vegetative morphology. Perennial; caespitose. Culms 15–150 cm high; herbaceous; nearly always unbranched above. Culm internodes hollow. Leaves not basally aggregated; non-auriculate. Leaf blades linear; narrow; 1–10 mm wide; flat, or folded, or rolled (convolute); not pseudopetiolate; without cross venation; persistent; rolled in bud, or once-folded in bud. Ligule an unfringed membrane (sometimes puberulent); not truncate; 2–8 mm long.

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

Inflorescence. Inflorescence paniculate; open (usually narrow); espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets not secund.

Female-fertile spikelets. Spikelets 8–25 mm long; compressed laterally; disarticulating above the glumes; disarticulating between the florets. Rachilla prolonged beyond the uppermost female-fertile floret; hairy; the rachilla extension with incomplete florets. Hairy callus present. Callus short (0.5–1.2 mm long); pointed.

Glumes two; very unequal (usually), or more or less equal; usually shorter than the spikelets; (the upper usually) shorter than the adjacent lemmas; pointed; awnless; carinate; similar (persistent, hyaline to scarious or firm and herbaceous). Lower glume 1–3 nerved. Upper glume 3(–5) nerved. Spikelets with incomplete florets. The incomplete florets distal to the female-fertile florets. The distal incomplete florets merely underdeveloped; awned.

Female-fertile florets 2–7. Lemmas decidedly firmer than the glumes (firmly membranous to leathery); not becoming indurated; usually incised; 2 lobed (usually), or 4 lobed; not deeply cleft (bidentate or bisetulose, rarely 4-dentate); awned. Awns 1; median; dorsal; from near the top, or from well down the back (but from above the middle); geniculate (usually), or non-geniculate; occasionally straight, or recurving; much longer than the body of the lemma (always?); entered by one vein. Lemmas hairy, or hairless; carinate to non-carinate (weakly keeled to dorsally rounded); 5–7 nerved. Palea present; relatively long; tightly clasped by the lemma; apically notched; awnless, without apical setae; not indurated (membranous); 2-nerved; 2-keeled. Lodicules present; 2; free; membranous; glabrous; toothed; not or scarcely vascularized. Stamens 3. Anthers not penicillate. Ovary apically hairy. Styles free to their bases. Stigmas 2.

Fruit, embryo and seedling. Fruit medium sized; longitudinally grooved; compressed laterally, or not noticeably compressed; with hairs confined to a terminal tuft. Hilum long-linear. Embryo small; not waisted. Endosperm liquid in the mature fruit; with lipid; containing compound starch grains. Embryo with an epiblast; with a negligible mesocotyl internode.

Seedling with a long mesocotyl; with a tight coleoptile. First seedling leaf with a well-developed lamina. The lamina narrow; 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 common. Subsidiaries low dome-shaped, or parallel-sided. Guard-cells overlapped by the interstomatals. Intercostal short-cells absent or very rare (none seen). 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.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with non-radiate chlorenchyma; without adaxial palisade. Leaf blade adaxially flat. Midrib conspicuous; with one bundle only. Bulliforms not present in discrete, regular adaxial groups (limited to midrib ‘hinges’). All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present. Sclerenchyma all associated with vascular bundles.

Phytochemistry. Leaves without flavonoid sulphates (1 species).

Cytology. Chromosome base number, x = 7. 2n = 14, 28, 42, 70, 81, 98, 112, 126, 133, and 147. 2, 4, 6, 10, 12, 14, 16, 18, 19, and 21 ploid. Chromosomes ‘large’. Haploid nuclear DNA content 3–5 pg (2 species). Mean diploid 2c DNA value 10.1 pg (pubescens).

Classification. Watson & Dallwitz (1994): Pooideae; Poodae; Aveneae. Soreng et al. (2015): Pooideae; Poodae; Poeae; Aveninae, or Airinae (Avenula p.p. only). About 90 species.

Distribution, phytogeography, ecology. Europe, Africa, Southeast Asia, North & South America.

Mesophytic to xerophytic, or helophytic (rarely); mostly species of open habitats. Dry hillsides, meadows, margins of woods.

Economic aspects. Important native pasture species: H. milanjianum in Kenya.

Rusts and smuts. Rusts — Puccinia. Taxonomically wide-ranging species: Puccinia graminis and Puccinia coronata. Smuts from Tilletiaceae and from Ustilaginaceae. Tilletiaceae — Urocystis. Ustilaginaceae — Ustilago.

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

Illustrations. • H. leoninum, as Avena: Kunth (1835). • H. hirtulum, general aspect: Gibbs Russell et al., 1990. • H. pratense (as Avena): Eng. Bot. (1872). • cf. H. alpinum (as Avena pratensis var. alpina), general aspect: Eng. Bot. (1872). • H. pubescens (as Avena): Eng. Bot. (1872)


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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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