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The grass genera of the world

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

Chaetobromus Nees

From the Greeek chaeto (bristle) and Bromus; the spikelets look like those of Bromus, but the lemmas have longer awns and bristles.

Type species: Type: C. involucratus (Schrad.) Nees).

Habit, vegetative morphology. Perennial; rhizomatous (sometimes), or stoloniferous, or caespitose, or decumbent. Culms (15–)30–80 cm high; herbaceous; to to 0.3 cm in diameter; branched above (but not profusely). Culm nodes black, glabrous. Culm internodes hollow. Plants unarmed. Young shoots intravaginal. Leaves not basally aggregated; non-auriculate (but hairy at the auricle positions). Sheath margins free. Leaf blades linear to linear-lanceolate; narrow; 1–5 mm wide; flat, or folded; not pseudopetiolate; without cross venation; persistent. Ligule a fringe of hairs.

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

Inflorescence. Inflorescence paniculate (rarely racemose, in depauperate plants); open, or contracted (sometimes with few spikelets); with capillary branchlets, or without capillary branchlets. Inflorescence with axes ending in spikelets. Inflorescence espatheate. Spikelet-bearing axes persistent. Spikelets solitary; not secund; pedicellate (the pedicels articulated).

Female-fertile spikelets. Spikelets 10–17 mm long; compressed laterally; falling with the glumes (the hairs on the persistent pedicel allowing the spikelet to move in only one direction); ultimately disarticulating between the florets (i.e., disarticulation occurring in the pedicel, above the glumes, and between the florets). Rachilla prolonged beyond the uppermost female-fertile floret; hairy; the rachilla extension with incomplete florets. Hairy callus present (the lowermost comprising the bearded upper part of pedicel and the rachilla below the glumes, plus a conventional one under each floret). Callus short (beneath each floret shorter to longer than the rachilla internode), or long (under the glumes); under each spikelet blunt (rounded or truncate).

Glumes two; more or less equal; exceeding the spikelets; long relative to the adjacent lemmas; free; hairy (puberulous), or hairless; scabrous (on the keels); pointed; awnless; carinate; similar (subherbaceous, with scarious margins, the upper narrower). Lower glume 5–11 nerved. Upper glume (3–)5–11 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets, or distal to the female-fertile florets. The distal incomplete florets if present, merely underdeveloped; awned. Spikelets without proximal incomplete florets (but the lower floret differing from the rest, being smaller, with shorter awn and setae, and fewer veins).

Female-fertile florets (2–)3–4(–6). Lemmas less firm than the glumes to similar in texture to the glumes (membranous); not becoming indurated; incised; 2 lobed; deeply cleft; awned, or awnless and awned (the L1 sometimes with a reduced awn or awnless). Awns 1, or 3; median, or median and lateral (the lateral lemma lobes sometimes being bristle-tipped); the median different in form from the laterals (when laterals present); from a sinus (mostly), or apical (sometimes, in the L1); median geniculate (and twisted below); hairless; much longer than the body of the lemma, or much shorter than the body of the lemma to about as long as the body of the lemma (in the lowest floret); entered by one vein. The lateral awns when present, shorter than the median (and straight). Awn bases median twisted. Lemmas hairy (mostly, pilose), or hairless (L1). The hairs not in tufts. Lemmas non-carinate; (5–)7 nerved (the lowest one), or 9 nerved (the rest). Palea present; relatively long; entire, or apically notched (then obscurely 3-notched); awnless, without apical setae; textured like the lemma; not indurated; 2-nerved; lorate to linear, 2-keeled. Lodicules present; 2; free; fleshy; ciliate (but without microhairs), or glabrous. Stamens 3. Anthers 5–6 mm long; not penicillate; without an apically prolonged connective. Ovary apically glabrous. Styles free to their bases (short). Stigmas 2; rusty red pigmented.

Fruit, embryo and seedling. Fruit tightly enclosed by lemma and palea, but free from both lemma and palea; lorate ellipsoid; longitudinally grooved; slightly compressed laterally. Hilum long-linear.

Ovule, embryology. Micropyle not noticeably oblique. Outer integument covering no more than the chalazal half of the ovule; more than two cells thick at the micropylar margin. Inner integument discontinuous distally; not thickened around the micropyle. Synergids haustorial (strongly developed); exhibiting large, globular starch grains.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells markedly different in shape costally and intercostally (the intercostals fusiform, the costals rectangular). Mid-intercostal long-cells fusiform; having markedly sinuous walls and having straight or only gently undulating walls (and the costals with sinuous walls). Microhairs absent (but panicoid type present adaxially). Stomata common (but apparently confined to two lateral files per intercostal zone); 36–42 microns long. Subsidiaries low dome-shaped, or parallel-sided. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; in cork/silica-cell pairs; silicified. Intercostal silica bodies often crescentic. Costal short-cells conspicuously in long rows. Costal silica bodies horizontally-elongated crenate/sinuous, or ‘panicoid-type’ (predominantly ‘panicoid type’, but the undulations sometimes amount to crenation).

Transverse section of leaf blade, physiology. C4 (in C. involucratus, according to carbon isotope ratio - H. Ziegler, pers. comm.), or C3 (seemingly, judging from the poor material seen of C. dregeanus, though the lateral cell count is low between all but a few bundles); XyMS+ (C. dregeanus). Mesophyll without adaxial palisade. Leaf blade adaxially flat (with marked abaxial ribs only). Midrib not readily distinguishable (the bundle and keel only very slightly larger); with one bundle only; with colourless mesophyll adaxially (in the form of a wide bundle-sheath extension). Bulliforms present in discrete, regular adaxial groups; in simple fans (the groups large). Many of the smallest vascular bundles unaccompanied by sclerenchyma (and these ‘inserted’ between the large veins, as in panicoid C4 leaves). Combined sclerenchyma girders absent (the large bundles with adaxial strands and girders, linked to the bundles by colourless sheath extensions). Sclerenchyma all associated with vascular bundles.

Special diagnostic feature. Pedicels articulated and bearded with long hairs at and above the joint. Female-fertile lemmas with a bent awn, the awn twisted below.

Cytology. Chromosome base number, x = 6. 2n = 12, 18, 36, 48, 52, 72.

Classification. Watson & Dallwitz (1994): Arundinoideae; Danthonieae. Soreng et al. (2015): Danthonioideae; Danthonieae. 1 species (C. involucratus).

Distribution, phytogeography, ecology. Southern Africa.

Xerophytic; halophytic, or glycophytic. Commonly coastal, generally on sandy soil.

References, etc. Leaf anatomical: this project.

Illustrations. • C. dregeanus: Gibbs Russell et al., 1990


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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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