| # The grass genera of the world |

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

### Aristida L.

From the Latin arista or the Greek aristos (bristle, or awn from an ‘ear’ of corn).

Threeawn grasses, Kerosene grasses.

Including Aristopsis Catasus, Arthratherum P. Beauv., Chaetaria P. Beauv., Curtopogon P. Beauv., Kielboul Adans., Moulinsia Raf., Streptachne R. Br., Trixostis Raf.

Habit, vegetative morphology. Annual, or perennial; caespitose. Culms 10–100(–180) cm high; herbaceous; branched above, or unbranched above. The branching dendroid (rarely), or fastigiate (sometimes), or suffrutescent, or simple. Culm nodes glabrous. Culm internodes solid, or hollow. Leaves mostly basal, or not basally aggregated (in annual species); non-auriculate. Leaf blades linear, or linear-lanceolate; narrow; flat, or rolled; without cross venation; persistent; rolled in bud, or once-folded in bud. Ligule present; a fringed membrane to a fringe of hairs. Contra-ligule present (of hairs), or absent.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets; exposed-cleistogamous, or chasmogamous; with hidden cleistogenes, or without hidden cleistogenes. The hidden cleistogenes in the leaf sheaths (when present).

Inflorescence. Inflorescence few spikeleted, or many spikeleted; paniculate; deciduous in its entirety, or not deciduous; open, or contracted. Rachides hollowed, or flattened, or winged, or neither flattened nor hollowed, not winged. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets not secund; pedicellate.

Female-fertile spikelets. Spikelets 4–30 mm long; compressed laterally to not noticeably compressed; disarticulating above the glumes. Rachilla terminated by a female-fertile floret. Hairy callus present. Callus long; pointed.

Glumes two; relatively large; very unequal to more or less equal; (at least the G2) about equalling the spikelets, or exceeding the spikelets; usually pointed; awned, or awnless; carinate; very dissimilar, or similar (membranous to papery). Lower glume 1 nerved. Upper glume 1 nerved (usually), or 3 nerved (rarely). Spikelets with female-fertile florets only; without proximal incomplete florets.

Female-fertile florets 1. Lemmas narrow, cylindrical; convolute, or not convolute; decidedly firmer than the glumes; becoming indurated to not becoming indurated (leathery to indurated); entire; awned. Awns triple or trifid, commonly with a basal column (usually), or not of the triple/trifid, basal column type (the column sometimes absent, the lateral branches sometimes reduced or absent); 1, or 3; apical; non-geniculate (at least, not geniculate in the normal sense); hairless (usually glabrous); much shorter than the body of the lemma to much longer than the body of the lemma; entered by several veins (usually with three veins in the column); deciduous, or persistent. Lemmas hairy (rarely), or hairless; glabrous, or scabrous; non-carinate; with a clear germination flap; 1–3 nerved. Palea present; conspicuous but relatively short, or very reduced (enclosed by the lemma); entire; awnless, without apical setae; thinner than the lemma (hyaline); not indurated; 1-nerved, or 2-nerved, or nerveless. Lodicules present, or absent; when present, 2; free; membranous; ciliate (rarely), or glabrous; not toothed; rather heavily vascularized. Stamens 1–3. Anthers 0.7–2 mm long; not penicillate. Ovary apically glabrous. Styles free to their bases. Stigmas 2; red pigmented, or brown.

Fruit, embryo and seedling. Fruit free from both lemma and palea; small to large (3–11 mm long); fusiform; compressed dorsiventrally, or not noticeably compressed. Hilum short, or long-linear. Embryo large; waisted, or not waisted. Endosperm hard; without lipid; containing compound starch grains. Embryo without an epiblast; with a scutellar tail; with an elongated mesocotyl internode; with one scutellum bundle. Embryonic leaf margins meeting.

Seedling with a short mesocotyl. First seedling leaf with a well-developed lamina. The lamina narrow; curved; 3–5 veined.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous (or somewhat obscure in some species). Papillae absent. Long-cells similar in shape costally and intercostally; of similar wall thickness costally and intercostally (usually fairly thick walled). Mid-intercostal long-cells rectangular (and narrow); having markedly sinuous walls. Microhairs present; elongated; clearly two-celled; panicoid-type; 48–81(–87) microns long; 5–9.6 microns wide at the septum. Microhair total length/width at septum 8.4–12. Microhair apical cells 24–44 microns long. Microhair apical cell/total length ratio 0.4–0.6. Stomata common; 22.5–36 microns long. Subsidiaries dome-shaped, or triangular, or dome-shaped and triangular. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common, or absent or very rare; in cork/silica-cell pairs (and solitaries); silicified (when paired), or not silicified (when solitary). Intercostal silica bodies variable in shape. Costal short-cells conspicuously in long rows, or neither distinctly grouped into long rows nor predominantly paired. Costal silica bodies horizontally-elongated crenate/sinuous, or horizontally-elongated smooth, or rounded, or saddle shaped, or tall-and-narrow, or crescentic, or oryzoid, or ‘panicoid-type’; when panicoid type, variously cross shaped to dumb-bell shaped, or nodular (often exhibiting elongated dumb-bells with enlarged, bulbous ends, and/or very large, nodular forms).

Transverse section of leaf blade, physiology. C4. The anatomical organization unconventional. Organization of PCR tissue Aristida type. Biochemical type NADP–ME (3 species); XyMS– (with double PCR sheaths). PCR cells without a suberised lamella. PCR cell chloroplasts ovoid; with well developed grana (in the outer sheath), or with reduced grana (in the inner sheath); centrifugal/peripheral (in the inner sheath), or centripetal (in the outer sheath). Mesophyll with radiate chlorenchyma; traversed by columns of colourless mesophyll cells (usually), or not traversed by colourless columns (e.g. in A. caput-medusae). Leaf blade with distinct, prominent adaxial ribs; with the ribs very irregular in sizes. Midrib conspicuous, or not readily distinguishable; with one bundle only. Bulliforms associated with colourless mesophyll cells to form deeply-penetrating fans (these often linked with colourless girders, the latter fully traversing or not). All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; nowhere forming ‘figures’. Sclerenchyma all associated with vascular bundles, or not all bundle-associated. The ‘extra’ sclerenchyma when present, in abaxial groups; abaxial-hypodermal, the groups continuous with colourless columns.

Culm anatomy. Culm internode bundles in three or more rings, or scattered.

Phytochemistry. Leaves without flavonoid sulphates (6 species). Leaf blade chlorophyll a:b ratio 4.46–4.65.

Cytology. Chromosome base number, x = 11 and 12. 2n = 22, 24, 36, 44, 48, and 66. 2, 4, and 6 ploid. Chromosomes ‘small’. Nucleoli persistent.

Classification. Watson & Dallwitz (1994): Arundinoideae; Aristideae. Soreng et al. (2015): Aristidoideae; Aristideae. 290 species.

Distribution, phytogeography, ecology. Temperate and subtropical.

Xerophytic.

Economic aspects. Significant weed species: (e.g.) A. adscensionis, A. dichotoma, A. longiseta, A. oligantha - the awns of many species can injure livestock. Important native pasture species: none - generally poor value, but various species of minor grazing value in dry regions.

Rusts and smuts. Rusts — Puccinia. Taxonomically wide-ranging species: Puccinia graminis and Puccinia aristidae. Smuts from Tilletiaceae and from Ustilaginaceae. Tilletiaceae — Tilletia. Ustilaginaceae — Sorosporium, Sphacelotheca, Tolyposporium, and Ustilago.

References, etc. Morphological/taxonomic: de Winter 1965; Lazarides 1980. Leaf anatomical: Metcalfe 1960; studied by us - A. adscensionis L., A. aemulans Melderis, A. brainii Melderis, A. congesta Roem. &amp; Schult., A. caput-medusae Domin, A. hordeacea Kunth, A. kenyensis Henr., A. meridionalis Henr., A. ramosa R. Br., A. rhiniochloa Hochst., A. scabrivalvis Hack., A. stipitata Hack., A. vestita Thunb.

Illustrations. • Aristida adscensionis, vegetative: Pan XingLai, China, (2023). • A. adscensionis, ears or heads: Pan XingLai, China, (2023). • A. adscensionis, reproductive organization: Pan XingLai, China, (2023). • A. adscensionis, fruit, embryo, seedling: Pan XingLai, China, (2023). • Aristida caerulescens: Bor, Flora of Iraq (1968). • Aristida congesta, general aspect: Gibbs Russell et al., 1990. • Aristida meridionalis: Gibbs Russell et al., 1990. • Aristida ramosa, inflorescence. • Spikelet of Aristida ramosa. • Spikelet forms (5 Australian species). • Spikelet forms (4 Australian species). • Lemmas of 8 Australian species: Gardner, 1952. • Abaxial epidermis of leaf blade of Aristida ramosa. • Abaxial epidermis of leaf blade. • Aristida biglandulosa, T.S. leaf blade: this project. • Aristida biglandulosa, T.S. leaf blade, fluorescence image (1): Hattersley, Watson and Osmond (1977). See Hattersley, Watson and Osmond (1977). • Aristida biglandulosa, T.S. leaf blade, fluorescence images (2): Hattersley, Watson and Osmond (1977). See Hattersley, Watson and Osmond (1977). • Leaf blade T.S. of Aristida behriana, fluorescence image: Hattersley, Watson and Osmond (1977). See Hattersley, Watson and Osmond (1977). • Aristida ramosa, T.S. of the leaf blade: this project. • Aristida ramosa leaf blade T.S. with immunofluorescent-labelled Rubisco: Hattersley, Watson and Osmond 1977. See Hattersley, Watson and Osmond (1977). Note the double PCR sheaths, with centripetal chloroplasts in the outer ones. • Germination in Aristida ramosa (1): this project. Aristida ramosa. Radicle emerging between raised flap and callus, shoot emerging between the overlapping lemma margins (lower left). Aristida ramosa. Raised germination flap, with radicle emerging. • Germination in Aristida ramosa (2): this project. Aristida ramosa. Radicle emerging between raised germination flap and callus, shoot (upper left) from between the rolled lemma margins.

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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