| # The grass genera of the world |

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

### Trisetum Pers.

From the Latin tri (three) and setum (bristle), alluding to lemmas with three awns.

Type species: Type: T. striatum (Lam.) Pers. ??.

Including Acrospelion Schult., Parvotrisetum Chrtek, Rupestrina Prov., Sennenia Sennen, Trisetaria Forssk., Trisetarium Poir.

Excluding Avellinia, Peyritschia

Habit, vegetative morphology. Annual (Trisetaria), or perennial; rhizomatous, or caespitose. Culms 4–150 cm high; herbaceous. Culm internodes hollow. Young shoots extravaginal. Leaves non-auriculate. Leaf blades linear; apically cucullate, or apically flat; narrow; 0.2–12 mm wide; flat, or rolled (convolute); without cross venation; persistent; rolled in bud. Ligule an unfringed membrane (sometimes puberulent or ciliolate); truncate, or not truncate; 1–1.5 mm long.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets all alike in sexuality. Plants outbreeding and inbreeding; exposed-cleistogamous, or chasmogamous.

Inflorescence. Inflorescence paniculate; open, or contracted; when contracted, spicate (but interrupted), or more or less irregular; espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets not secund; pedicellate.

Female-fertile spikelets. Spikelets 2.4–9 mm long; compressed laterally; disarticulating above the glumes, or falling with the glumes, or not disarticulating. Rachilla prolonged beyond the uppermost female-fertile floret; hairy (usually), or hairless (rarely). Hairy callus usually present. Callus short.

Glumes two; relatively large; very unequal to more or less equal; shorter than the spikelets, or about equalling the spikelets; shorter than the adjacent lemmas, or long relative to the adjacent lemmas; pointed (acute or acuminate); awnless; carinate; similar. Lower glume 1–3 nerved. Upper glume 1–5 nerved. Spikelets with female-fertile florets only, or with incomplete florets. The incomplete florets distal to the female-fertile florets. Spikelets without proximal incomplete florets.

Female-fertile florets (1–)2–5(–12). Lemmas similar in texture to the glumes; not becoming indurated; incised; 2 lobed (bifid or with two setae); usually conspicuously awned (by contrast with Koeleria, but rarely awnless). Awns when present, 1, or 3; median, or median and lateral (via setae from the lobes); the median different in form from the laterals (when laterals present); dorsal (or ‘subterminal’); from near the top, or from well down the back; non-geniculate, or geniculate; much shorter than the body of the lemma to much longer than the body of the lemma; entered by one vein. The lateral awns when present, shorter than the median. Lemmas hairless; carinate; 3–7 nerved. Palea present; relatively long; gaping; with apical setae (the two nerves ending as bristle tips); thinner than the lemma; 2-nerved; 2-keeled. Lodicules present; 2; free; membranous; ciliate, or glabrous; toothed, or not toothed; not or scarcely vascularized. Stamens 3. Anthers 0.3–4.5 mm long; not penicillate; without an apically prolonged connective. Ovary usually apically glabrous. Styles free to their bases. Stigmas 2; white.

Fruit, embryo and seedling. Fruit small, or medium sized; dull; slightly compressed laterally. Hilum short. Embryo small; not waisted. Endosperm liquid in the mature fruit; 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 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, or markedly different in shape costally and intercostally; of similar wall thickness costally and intercostally, or differing markedly in wall thickness costally and intercostally. Mid-intercostal long-cells rectangular, or fusiform; having markedly sinuous walls, or having straight or only gently undulating walls. Microhairs absent. Stomata common; (30–)33–41(–42) microns long. Subsidiaries parallel-sided. Guard-cells overlapped by the interstomatals. Intercostal short-cells common, or absent or very rare; not paired; not silicified. Costal zones with short-cells. Costal short-cells conspicuously in long rows, or predominantly paired, or neither distinctly grouped into long rows nor predominantly paired. Costal silica bodies horizontally-elongated crenate/sinuous, or horizontally-elongated smooth; not sharp-pointed.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with non-radiate chlorenchyma. Leaf blade with the ribs more or less constant in size, or with the ribs very irregular in sizes. Midrib not readily distinguishable; with one bundle only. Bulliforms present in discrete, regular adaxial groups; in simple fans. All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present. Sclerenchyma all associated with vascular bundles.

Phytochemistry. Leaves without flavonoid sulphates (1 species).

Special diagnostic feature. Panicle loose, or if dense then interrupted, neither cylindrical nor ovoid: awns usually present, usually twisted, usually distinctly dorsal, conspicuous if inflorescence compact.

Cytology. Chromosome base number, x = 6 and 7. 2n = 12, 14, 24, 28, 42, and 56. 2, 4, 6, and 8 ploid. Chromosomes ‘large’.

Classification. Watson & Dallwitz (1994): Pooideae; Poodae; Aveneae. Soreng et al. (2015): Pooideae; Poodae; Poeae; Aveninae, or Poinae (Parvotrisetum only). About 85 species.

Distribution, phytogeography, ecology. North &. South temperate.

Commonly adventive. Mesophytic, or xerophytic; mostly species of open habitats; glycophytic. Meadows, mountain slopes, upland grasslands, weedy places.

Economic aspects. Cultivated fodder: T. flavescens. Important native pasture species: T. flavescens, T. spicatum, T. wolfii etc.

Hybrids. Integeneric hybrids with Koeleria (×Trisetokoeleria Tsvelev), Sphenopholis.

Rusts and smuts. Rusts — Puccinia. Taxonomically wide-ranging species: Puccinia graminis, Puccinia coronata, Puccinia striiformis, Puccinia brachypodii, Puccinia poarum, Puccinia hordei, Puccinia recondita, ‘Uromyces’ dactylidis, and Puccinia monoica. Smuts from Tilletiaceae and from Ustilaginaceae. Tilletiaceae — Entyloma and Urocystis. Ustilaginaceae — Ustilago.

References, etc. Morphological/taxonomic: Koch 1979. Leaf anatomical: Metcalfe 1960; studied by us - Trisetum spicatum (L.) Richter.

Special comments. See description of Koeleria for comments on the unsatisfactory taxonomic situation around Koeleria and Trisetum (involving Trisetaria, Graphephorum, Rostraria, Lophochloa and Peyritschia).

Illustrations. • Trisetum antarcticum: Hooker, Fl. Novae-Zelandiae (1853). • Trisetum flavescens: Bor, Flora of Iraq (1968). • Ttisetum flavescens (as Avena): Eng. Bot. (1872). • Trisetum lineare (~Trisetaria linearis): Bor, Fl. of Iraq (1968). • Trisetum spicatum, as T. toluccense: Kunth (1835). • Inflorescence of Trisetum spicatum subsp. australiense. • Opened spikelet of T. spicatum subsp. australiense. • Trisetum spicatum, abaxial epidermis of leaf blade. • Trisetum spicatum, transverse section of leaf blade.

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: 15th June 2025. delta-intkey.com’.

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