| # The grass genera of the world |

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

### Tristachya Nees

Including Apochaete (C. E. Hubbard) Phipps, Dolichochaete Phipps, Loudetia A. Br., Monopogon Presl, Muantijamvella Phipps, Veseyochloa Phipps

Excluding Isalus

Habit, vegetative morphology. Annual (rarely), or perennial; caespitose. Culms 15–270 cm high; herbaceous; tuberous, or not tuberous. Leaf blades broad, or narrow; flat, or rolled (then involute or convolute, often rigid); not pseudopetiolate. Ligule a fringe of hairs.

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

Inflorescence. Inflorescence paniculate, or a single raceme (of triads); open, or contracted; espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets usually in triplets (the triads terminating the panicle branches, sometimes in unequally pedicelled groups of 2–3); not secund; pedicellate (the pedicels within a triad usually connate).

Female-fertile spikelets. Spikelets 10–45 mm long (often large); brown; compressed laterally to not noticeably compressed (?); disarticulating above the glumes; disarticulating between the florets (disarticulating easily between L1 and L2, less easily between G2 and L1). Rachilla terminated by a female-fertile floret (rarely prolonged?); hairless. Hairy callus present. Callus long; narrowly conical, pointed (usually pungent, sometimes narrowly obtuse in T. huillensis).

Glumes two; more or less equal; long relative to the adjacent lemmas; free; hairy (sometimes with cushion-based hairs), or hairless (glabrous); pointed, or not pointed; awnless (obtuse, or lanceolate to acuminate, or rostrate); non-carinate (flattened or convex); similar. Lower glume 3 nerved, or 5 nerved (rarely). Upper glume 3 nerved, or 5 nerved (rarely). Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. Spikelets with proximal incomplete florets. The proximal incomplete florets 1; paleate. Palea of the proximal incomplete florets fully developed (narrow, two keeled). The proximal incomplete florets male. The proximal lemmas awnless (similar to G2); (3–)5–9 nerved; exceeded by the female-fertile lemmas; less firm than the female-fertile lemmas to similar in texture to the female-fertile lemmas (papery to leathery); not becoming indurated.

Female-fertile florets 1. Lemmas similar in texture to the glumes to decidedly firmer than the glumes (leathery to cartilaginous); not becoming indurated; incised; 2 lobed (the lobes obtuse, pointed or aristulate); not deeply cleft; awned. Awns 1; median; from a sinus (from between the lobes); geniculate; hairless (scabrid), or hairy; much longer than the body of the lemma; deciduous (usually), or persistent (T. bicrinita). Lemmas hairy (usually), or hairless. The hairs in tufts (rarely, with tufts at the bases of the lobes), or not in tufts. Lemmas non-carinate; having the margins inrolled against the palea (the latter enclosed, save at its summit); with a clear germination flap; 5–7 nerved. Palea present; awnless, without apical setae; 2-nerved; 2-keeled. Palea keels wingless (thickened, but not wing-shaped). Lodicules present; 2; free; fleshy (narrowly cuneate). Stamens 3 (usually?). Ovary apically hairy. Styles free to their bases. Stigmas 2.

Fruit, embryo and seedling. Fruit free from both lemma and palea (but embraced by lemma and palea); longitudinally grooved. Hilum long-linear. Embryo large.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; panicoid-type; (48–)54–87(–90) microns long; 6–7.2 microns wide at the septum. Microhair total length/width at septum 10.9–15. Microhair apical cells 23–30 microns long (T. hispida), or 40–47 microns long (T. bequartii). Microhair apical cell/total length ratio 0.45–0.54. Stomata absent or very rare (e.g. in T. bequartii), or common. Subsidiaries dome-shaped (mostly, low), or triangular. Intercostal short-cells common (fairly); in cork/silica-cell pairs (and solitary). Costal zones with short-cells. Costal short-cells conspicuously in long rows. Costal silica bodies ‘panicoid-type’; mostly dumb-bell shaped; not sharp-pointed.

Transverse section of leaf blade, physiology. C4; XyMS–. PCR cell chloroplasts centrifugal/peripheral. Mesophyll traversed by columns of colourless mesophyll cells, or not traversed by colourless columns (in T. bequartii, there are broad zones of colourless tissue both adaxially and abaxially, but the vascular bundles are embedded in a continuous central zone of photosynthetic tissue). Leaf blade with distinct, prominent adaxial ribs to adaxially flat. Midrib conspicuous, or not readily distinguishable; with one bundle only, or having a conventional arc of bundles; with colourless mesophyll adaxially. Bulliforms present in discrete, regular adaxial groups; in simple fans, or associated with colourless mesophyll cells to form deeply-penetrating fans (sometimes traversing to the abaxial epidermis); associating with colourless mesophyll cells to form arches over small vascular bundles, or nowhere involved in bulliform-plus-colourless mesophyll arches. Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders present, or absent (e.g. T. bequartii); forming ‘figures’ (when present). Sclerenchyma all associated with vascular bundles.

Special diagnostic feature. The lower glume exceeding the female-fertile lemma.

Cytology. Chromosome base number, x = 10 and 12. 2n = 24 and 40.

Classification. Watson & Dallwitz (1994): Panicoideae; Panicodae; Arundinelleae. Soreng et al. (2015): Panicoideae; Tristachyideae. About 20 species.

Distribution, phytogeography, ecology. Tropical and southern Africa, Madagascar, tropical America.

Helophytic to xerophytic; shade species and species of open habitats; glycophytic. Grassland and savanna, woodland and floodplains, wet to dry soils.

Rusts and smuts. Rusts — Puccinia. Smuts from Ustilaginaceae. Ustilaginaceae — Sphacelotheca and Tolyposporium.

References, etc. Leaf anatomical: Metcalfe 1960; studied by us - T. bequartii de Willd.

Illustrations. • Tristachya leiostachya: Kunth (1835). • T. leucothrix: Wood, Natal Plants 2 (1904). • General aspect (T. leucothrix): Gibbs Russell et al., 1990. • Triplet of spikelets (Tristachya bequartii). Tristachya bequartii. Triplet teminating a panicle branch, the glumes with tubercle based hairs. • Abaxial epidermis of leaf blade of Tristachya bequartii: this project. • TS leaf blade of Tristachya bequartii: 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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