| # The grass genera of the world |

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

### Tridens Roem. & Schult.

From the Latin tria (thrice) and dens (tooth), referring to three-toothed lemmas.

Including Antonella Caro, Gossweilerochloa Renvoize, Tricuspis P. Beauv., Windsoria Nutt

Excluding Erioneuron, Tridentopsis

Habit, vegetative morphology. Perennial; rhizomatous to stoloniferous (rarely), or caespitose. Culms 5–160 cm high; herbaceous; branched above, or unbranched above. The branching simple. Culm nodes glabrous. Culm internodes hollow. Young shoots intravaginal. The shoots not aromatic. Leaves mostly basal, or not basally aggregated; non-auriculate. Leaf blades linear, or linear-lanceolate; narrow; setaceous, or not setaceous; flat, or folded, or rolled; without abaxial multicellular glands; not pseudopetiolate; without cross venation; rolled in bud. Ligule present; a fringed membrane, or a fringe of hairs.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets; exposed-cleistogamous; without hidden cleistogenes.

Inflorescence. Inflorescence paniculate; open to contracted (very variable); capitate to spicate; with capillary branchlets, or without capillary branchlets; espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets not secund; pedicellate.

Female-fertile spikelets. Spikelets 4–15 mm long; compressed laterally; disarticulating above the glumes; disarticulating between the florets; with conventional internode spacings. Rachilla prolonged beyond the uppermost female-fertile floret; the rachilla extension with incomplete florets. Hairy callus present, or absent. Callus short; blunt.

Glumes two; more or less equal; shorter than the spikelets; shorter than the adjacent lemmas, or long relative to the adjacent lemmas; pointed (acute to acuminate); awnless; carinate; similar (membranous, often thin). Lower glume 1 nerved. Upper glume 1–3(–7) nerved. Spikelets with incomplete florets. The incomplete florets distal to the female-fertile florets. The distal incomplete florets merely underdeveloped. Spikelets without proximal incomplete florets.

Female-fertile florets 3–12. Lemmas not becoming indurated; incised; 2 lobed; deeply cleft, or not deeply cleft (from minutely emarginate or toothed, to deeply and obtusely 2-lobed); mucronate to awned (the midnerve usually excurrent as a mucro or short awn, the laterals also often excurrent). Awns when present, 1, or 3; median, or median and lateral; the median similar in form to the laterals (when laterals present); from a sinus; non-geniculate; much shorter than the body of the lemma; entered by one vein. The lateral awns shorter than the median. Lemmas hairy (on the nerves, at least below); carinate to non-carinate; without a germination flap; 3 nerved. Palea present; relatively long (widened and bowed below); apically notched; awnless, without apical setae; not indurated; 2-nerved; 2-keeled. Palea keels wingless; glabrous to hairy. Lodicules present; 2; free; fleshy; glabrous. Stamens 3. Anthers not penicillate; without an apically prolonged connective. Ovary apically glabrous. Styles free to their bases. Stigmas 2; red pigmented.

Fruit, embryo and seedling. Fruit small to medium sized; dull brown; ellipsoid; compressed dorsiventrally; smooth. Hilum short (less than half the fruit length). Pericarp fused. Embryo small (about 2/5 of the grain length). Endosperm hard; without lipid. Embryo with an epiblast; with a scutellar tail; with an elongated mesocotyl internode. Embryonic leaf margins meeting.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells similar in shape costally and intercostally (the costals smaller, narrower); of similar wall thickness costally and intercostally. Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; elongated; clearly two-celled; chloridoid-type (but peculiar, the basal cell seemingly expanded below into a ‘foot’). Microhair apical cell wall of similar thickness/rigidity to that of the basal cell. Microhairs 18–21 microns long. Microhair basal cells 15 microns long. Microhairs 9–10.5 microns wide at the septum. Microhair total length/width at septum 1.7–2.3. Microhair apical cells 5.4–7.5 microns long. Microhair apical cell/total length ratio 0.26–0.42. Stomata common; 24–29 microns long. Subsidiaries dome-shaped (mostly), or triangular (a few, almost). Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common (fairly); not paired (solitary); not silicified. Intercostal silica bodies absent. A few intercostal prickles present. Crown cells absent. Costal zones with short-cells. Costal short-cells conspicuously in long rows. Costal silica bodies present in alternate cell files of the costal zones; saddle shaped (abundant), or ‘panicoid-type’; when panicoid type, cross shaped, or dumb-bell shaped, or nodular (variously incomplete); not sharp-pointed.

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

C4; biochemical type PCK; XyMS+. PCR sheath outlines uneven. PCR sheaths of the primary vascular bundles interrupted; interrupted abaxially only. PCR sheath extensions absent. PCR cells with a suberised lamella. PCR cell chloroplasts ovoid; with well developed grana; centrifugal/peripheral. Mesophyll with radiate chlorenchyma; not traversed by colourless columns (though some of the bulliform-plus-colourless groups come close to traversing). Leaf blade adaxially flat. Midrib conspicuous (by virtue of a larger bundle with heavy abaxial sclerenchyma, and adaxial colourless tissue); with one bundle only; with colourless mesophyll adaxially. Bulliforms present in discrete, regular adaxial groups; associated with colourless mesophyll cells to form deeply-penetrating fans (these deeply penetrating). All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present (with all the main bundles); forming ‘figures’ (midrib with an anchor, the rest with anchors or I’s). Sclerenchyma all associated with vascular bundles. The lamina margins with fibres.

Phytochemistry. Leaf blade chlorophyll a:b ratio 3.83–3.87.

Cytology. Chromosome base number, x = 8, or 10 (?). 2n = 16, 32, 40, and 72. 2, 4, 5, and 9 ploid.

Classification. Watson & Dallwitz (1994): Chloridoideae; main chloridoid assemblage. Soreng et al. (2015): Chloridoideae; Cynodonteae; Pappophorinae. 18 species.

Distribution, phytogeography, ecology. Eastern and southern U.S.A. and Mexico.

Species of open habitats. Meadows, plains, open woodland.

Rusts and smuts. Rusts — Puccinia. Taxonomically wide-ranging species: Puccinia aristidae. Smuts from Ustilaginaceae. Ustilaginaceae — Ustilago.

References, etc. Leaf anatomical: Metcalfe 1960; studied by us - T. braziliensis Nees.

Illustrations. • Tridens albescens, as Triodia: Vasey (1891), Ill. N. American Grasses I(2). • Tridens flavus (as Uralepis cuprea): Kunth (1835), Dist. méthodique de la famille Graminées. • Tridens brasiliensis: Nicora & Rúgolo de Agrasar (1987). • Tridens brasiliensis, abaxial epidermis of leaf blade: this project. • Tridens brasiliensis, TS 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: 15th June 2025. delta-intkey.com’.

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