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The grass genera of the world

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

Arthraxon P. Beauv.

From the Greek arthron (joint) and axon (axis), alluding to jointed rachides.

Including Alectoridia A. Rich., Batratherum Nees, Lasiolytrum Steud., Lucaea Kunth, Pleuroplitis Trin.

Habit, vegetative morphology. Annual, or perennial (often trailing); decumbent. Culms 10–100 cm high; herbaceous; branched above, or unbranched above; tuberous, or not tuberous. Culm nodes glabrous. Culm internodes solid, or hollow. Young shoots intravaginal. Leaves not basally aggregated; non-auriculate. Leaf blades linear-lanceolate to ovate-lanceolate; broad, or narrow; cordate; without cross venation; persistent. Ligule a fringed membrane (short).

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets of sexually distinct forms on the same plant (usually); hermaphrodite, or hermaphrodite and male-only, or hermaphrodite and sterile; overtly heteromorphic, or homomorphic.

Inflorescence. Inflorescence of spicate main branches; usually subdigitate, or non-digitate (when the racemes solitary). Primary inflorescence branches (1–)2–30. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs (but the inflorescences terminal and/or axillary). Spikelet-bearing axes spikes (rarely), or ‘racemes’ (usually); clustered; with very slender rachides; disarticulating; disarticulating at the joints. ‘Articles’ linear; without a basal callus-knob; not appendaged; disarticulating transversely. Spikelets solitary (rarely), or paired; secund; sessile (the pedicelled spikelets sometimes completely suppressed), or sessile and pedicellate (but the latter often much reduced); consistently in ‘long-and-short’ combinations, or not in distinct ‘long-and-short’ combinations; when paired in pedicellate/sessile combinations. Pedicels of the ‘pedicellate’ spikelets (when present) free of the rachis. The ‘shorter’ spikelets hermaphrodite. The ‘longer’ spikelets male-only, or sterile.

Female-sterile spikelets. The pedicelled spikelets variable in form, male, sterile or vestigial, reduced to a sometimes microscopic pedicel, or totally suppressed.

Female-fertile spikelets. Spikelets 2–6.5 mm long; compressed laterally, or compressed dorsiventrally; falling with the glumes. Rachilla terminated by a female-fertile floret. Hairy callus absent.

Glumes two; more or less equal; long relative to the adjacent lemmas; without conspicuous tufts or rows of hairs; awnless; very dissimilar (the lower often leathery, rounded on the back, the upper less firm, laterally compressed). Lower glume more or less two-keeled, or not two-keeled; convex on the back; not pitted; rugose to prickly (sometimes with lateral rows of tubercles or spines); 5–20 nerved. Upper glume 1–5 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. The proximal incomplete florets 1; epaleate; sterile. The proximal lemmas awnless; 0–1 nerved; more or less equalling the female-fertile lemmas; similar in texture to the female-fertile lemmas (membranous); not becoming indurated.

Female-fertile florets 1. Lemmas ovate-lanceolate; less firm than the glumes (hyaline); not becoming indurated; entire, or incised; pointed to blunt (acute to erose); when incised, not deeply cleft; awned (nearly always), or awnless (A. submuticus). Awns 1; median; dorsal; from well down the back (near the base); geniculate; hairless (glabrous); much shorter than the body of the lemma to much longer than the body of the lemma. Lemmas hairless; non-carinate (rounded on the back); without a germination flap; 1 nerved, or 3 nerved. Palea present, or absent; when present, very reduced; entire (triangular, subacute); awnless, without apical setae; membranous; not indurated (hyaline); nerveless; keel-less. Lodicules present; 2; free; fleshy; glabrous. Stamens 2–3. Anthers not penicillate. Ovary apically glabrous. Styles free to their bases. Stigmas 2.

Fruit, embryo and seedling. Fruit free from both lemma and palea; fusiform, or ellipsoid; slightly compressed laterally. Hilum short. Embryo large. Endosperm hard; without lipid; containing compound starch grains. Embryo with an elongated mesocotyl internode.

Seedling with a long mesocotyl. First seedling leaf with a well-developed lamina. The lamina broad; supine.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present (though infrequent and often poorly defined in A. hispidulus). Intercostal papillae not over-arching the stomata; consisting of one oblique swelling per cell (at least in A. hispidulus). Long-cells markedly different in shape costally and intercostally (the costals much narrower, the intercostals broad); of similar wall thickness costally and intercostally (quite thin walled). Intercostal zones with typical long-cells (though these rather short, in places). Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; panicoid-type; (18–)22–37(–39) microns long; 4.5–6.3 microns wide at the septum. Microhair total length/width at septum 6.2–6.7. Microhair apical cells 10–21 microns long. Microhair apical cell/total length ratio 0.5–0.61. Stomata common; 30–33 microns long. Subsidiaries dome-shaped and triangular. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells absent or very rare. Costal short-cells conspicuously in long rows. Costal silica bodies ‘panicoid-type’; butterfly shaped, or dumb-bell shaped.

Transverse section of leaf blade, physiology. C4; XyMS–. Mesophyll with radiate chlorenchyma, or with non-radiate chlorenchyma (rarely). Leaf blade adaxially flat. Midrib conspicuous; with one bundle only. Bulliforms in irregular groups. Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders absent (adaxial strands combined with abaxial girders or strands, or the bundles with abaxial sclerenchyma only). Sclerenchyma all associated with vascular bundles.

Culm anatomy. Culm internode bundles in one or two rings, or in three or more rings.

Phytochemistry. Leaves without flavonoid sulphates (1 species).

Cytology. Chromosome base number, x = 9 and 10. 2n = 18, 20, 36, and 40. 2 and 4 ploid. Chromosomes ‘small’.

Classification. Watson & Dallwitz (1994): Panicoideae; Andropogonodae; Andropogoneae; Andropogoninae. Soreng et al. (2015): Panicoideae; Andropogonodae; Andropogoneae; Arthraxoninae. 7 species.

Distribution, phytogeography, ecology. Tropical Africa, Madagascar, Mauritius, Indomalayan region to Japan.

Helophytic to mesophytic; shade species, or species of open habitats; glycophytic.

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

References, etc. Morphological/taxonomic: Van Welzen 1981. Leaf anatomical: Metcalfe 1960; studied by us - A. hispidus (Thunb.) Makino.

Illustrations. • A. lancifolius: Rose Innes, Ghana Grasses (1977). • A. hispidus, as A. ciliare: P. Beauv. (1812). • A. hispidus: Hitchcock and Chase (1950)

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.’.