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The grass genera of the world

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

Ctenium Panzer

From the Greek ktenion (a little comb), alluding to a pectinate arrangement of spikelets.

Including Aplocera Raf., Campuloa Desv., Campulosus Desv., Monathera Raf., Monocera Elliott, Triatherus Raf.

Habit, vegetative morphology. Perennial (usually), or annual; densely caespitose. Culms 40–100 cm high; herbaceous. Plants unarmed. The shoots aromatic (some species - e.g. C. elegans - smelling strongly of turpentine), or not aromatic. Leaf blades linear; narrow; setaceous, or not setaceous; flat, or rolled (convolute); without abaxial multicellular glands; without cross venation. Ligule a fringed membrane (very short).

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets all alike in sexuality.

Inflorescence. Inflorescence a single spike, or of spicate main branches; non-digitate, or digitate (when of more than one spike). Primary inflorescence branches 1–3 (the spikes pectinate, usually curved). Rachides hollowed and flattened (crescentic in section). Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes spikes; with substantial rachides; persistent. Spikelets solitary; secund; biseriate (along the midrib of the rachis).

Female-fertile spikelets. Spikelets 4–9 mm long; adaxial; compressed laterally; disarticulating above the glumes; not disarticulating between the florets. Rachilla prolonged beyond the uppermost female-fertile floret; the rachilla extension with incomplete florets.

Glumes present; two; very unequal; long relative to the adjacent lemmas; G2 tubercled on the nerves; pointed (acute); G2 awned (shortly awn-tipped, and with a spreading awn from the middle of its back); carinate (G1), or non-carinate (G2 flat or rounded); very dissimilar (G2 larger, firmer, awned). Lower glume 1 nerved. Upper glume 2 nerved, or 3 nerved. Spikelets with incomplete florets. The incomplete florets both distal and proximal to the female-fertile florets. The distal incomplete florets merely underdeveloped (male or barren). Spikelets with proximal incomplete florets. The proximal incomplete florets 2; paleate (the paleas hyaline, variously reduced), or epaleate; male, or sterile. The proximal lemmas awned (from just below tip); similar in texture to the female-fertile lemmas (thin, white).

Female-fertile florets 1. Lemmas less firm than the glumes (i.e., than G2 - membranous); not becoming indurated; entire; pointed, or blunt; awned. Awns 1; median; dorsal; from near the top; non-geniculate; hairless (scabrid); about as long as the body of the lemma to much longer than the body of the lemma; entered by one vein. Lemmas hairy (ciliate on nerves); carinate; 3 nerved. Palea present; relatively long; not indurated; 2-nerved; 2-keeled. Lodicules present; 2; free; fleshy; glabrous. Stamens 3 (2 in male florets). Ovary apically glabrous. Styles free to their bases. Stigmas 2.

Fruit, embryo and seedling. Fruit free from both lemma and palea (embraced); ellipsoid. Hilum short. Pericarp fused. Embryo large; with an epiblast (rudimentary); with a scutellar tail; with an elongated mesocotyl internode. Embryonic leaf margins meeting.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; more or less spherical to elongated; clearly two-celled; chloridoid-type (basal cell rather long). Microhair apical cell wall of similar thickness/rigidity to that of the basal cell. Microhairs 18–24 microns long. Microhair basal cells 13–18 microns long. Microhairs 9–10.5 microns wide at the septum. Microhair total length/width at septum 2–2.7. Microhair apical cells (7.5–)9 microns long. Microhair apical cell/total length ratio 0.38–0.43. Stomata common; 19.5–24 microns long. Subsidiaries triangular. Intercostal short-cells common (often paired with prickle bases). Intercostal silica bodies absent. Prickles very abundant in C. polystachyum. Crown cells absent. Costal silica bodies present in alternate cell files of the costal zones; ‘panicoid-type’; cross shaped, or dumb-bell shaped, or nodular.

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

C4; XyMS+. PCR sheath outlines even. PCR sheaths of the primary vascular bundles interrupted; interrupted abaxially only. PCR sheath extensions present. Maximum number of extension cells 1, or 2. PCR cell chloroplasts centripetal. Mesophyll with radiate chlorenchyma; traversed by columns of colourless mesophyll cells; with arm cells (these rather conspicuous, in the material of C. polystachyum seen), or without arm cells (?). Leaf blade with distinct, prominent adaxial ribs to ‘nodular’ in section; with the ribs more or less constant in size. Midrib not readily distinguishable; with one bundle only. Bulliforms present in discrete, regular adaxial groups; associated with colourless mesophyll cells to form deeply-penetrating fans (these incorporated in the colourless columns). All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present (with most bundles). Sclerenchyma all associated with vascular bundles.

Cytology. Chromosome base number, x = 9. 2n = 18, 36, 54, and 160. 2 ploid, or 4 ploid, or 6 ploid, or 16 ploid (?).

Classification. Watson & Dallwitz (1994): Chloridoideae; main chloridoid assemblage. Soreng et al. (2015): Chloridoideae; Cynodonteae; Cteniinae. 20 species.

Distribution, phytogeography, ecology. Tropical and subtropical America and Africa.

Species of open habitats. Savanna.

References, etc. Leaf anatomical: Metcalfe 1960; studied by us - C. polystachyum Balansa.

Illustrations. • C. aromaticum, as C. americanum: Lamson-Scribner (1890). • C. elegans: Kunth (1835). • C. polystachyum: Gibbs Russell et al., 1990. • C. polystachyum, T.S. 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: 11th December 2017.’.