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The grass genera of the world

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

Microstegium Nees

From the Greek micros (small) and stege (a roof or cover), alluding to small lemmas.

Including Coelarthron Hook.f., Ephebopogon Steud., Leptatherum Nees, Nemastachys Steud., Psilopogon Hochst.

Excluding Ischnochloa

Habit, vegetative morphology. Creeping or rambling annual, or perennial; decumbent. Culms 30–60 cm high; herbaceous; branched above, or unbranched above. The branching simple. Leaves not basally aggregated; non-auriculate. Leaf blades linear to lanceolate; broad, or narrow; flat; pseudopetiolate to not pseudopetiolate; persistent. Ligule an unfringed membrane.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets usually all alike in sexuality (rarely, the pedicellate spikelet male); overtly heteromorphic to homomorphic (the pedicelled member with a less concave lower glume, and sometimes slightly smaller). Plants exposed-cleistogamous, or chasmogamous.

Inflorescence. Inflorescence of spicate main branches (flexuous, fragile racemes, these not villous); digitate, or subdigitate (to scattered on a short axis), or non-digitate (sometimes solitary). Primary inflorescence branches 1–25. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes with very slender rachides; disarticulating; disarticulating at the joints. ‘Articles’ linear, or non-linear (then clavate or pyriform); not appendaged; disarticulating transversely; densely long-hairy (rarely, villous), or somewhat hairy, or glabrous. Spikelets paired; sessile and pedicellate; consistently in ‘long-and-short’ combinations; in pedicellate/sessile combinations. Pedicels of the ‘pedicellate’ spikelets free of the rachis. The ‘shorter’ spikelets hermaphrodite. The ‘longer’ spikelets hermaphrodite.

Female-fertile spikelets. Spikelets compressed dorsiventrally; falling with the glumes (the pedicelled spikelet falling from its pedicel, the sessile falling with the adjacent internode and pedicel). Rachilla terminated by a female-fertile floret. Hairy callus present. Callus short.

Glumes two; more or less equal; long relative to the adjacent lemmas; awned (the upper, sometimes), or awnless; very dissimilar (the lower bicarinate and chanelled, the upper laterally compressed, naviculate). Lower glume two-keeled (via sharply inflexed margins, the keels not winged); sulcate on the back (with a deep groove or broadly concave median channel); not pitted; relatively smooth; 4–6 nerved. Upper glume 3 nerved. Spikelets with incomplete florets (though these are often more or less suppressed in some spikelets). The incomplete florets (when present) proximal to the female-fertile florets. Spikelets with proximal incomplete florets (often very reduced). The proximal incomplete florets 1; paleate, or epaleate. Palea of the proximal incomplete florets reduced. The proximal incomplete florets sterile. The proximal lemmas exceeded by the female-fertile lemmas to decidedly exceeding the female-fertile lemmas; similar in texture to the female-fertile lemmas (hyaline); not becoming indurated.

Female-fertile florets 1. Lemmas less firm than the glumes (hyaline or membranous); not becoming indurated; incised; 2 lobed (or 2-toothed); deeply cleft to not deeply cleft (usually bidentate to bifid, often minute); awned (nearly always), or awnless (rarely). Awns 1; median; usually from a sinus; geniculate; hairless (glabrous); much longer than the body of the lemma. Lemmas hairless; non-carinate. Palea present, or absent; relatively long, or conspicuous but relatively short to very reduced (always small, but sometimes exceeding the body of the L2); not indurated (hyaline); nerveless. 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. Hilum short. Embryo large. Endosperm hard; containing only simple starch grains. Embryo with an elongated mesocotyl internode.

Seedling with a long mesocotyl.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present. Intercostal papillae not over-arching the stomata; several per cell (at least in M. spectabile, where the papillae are mostly confined to costal long-cells and those adjoining the costal zones). Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; panicoid-type; 42–54 microns long; 6.6–7.5 microns wide at the septum. Microhair total length/width at septum 6.4–8.1. Microhair apical cells (18–)20–26(–32) microns long. Microhair apical cell/total length ratio 0.42–0.53. Stomata common; 28.5–34.5 microns long. Subsidiaries triangular (mostly), or dome-shaped (some,low). Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells absent or very rare. Prickle bases abundant. Costal short-cells conspicuously in long rows. Costal silica bodies ‘panicoid-type’; cross shaped, or butterfly shaped, or dumb-bell shaped, or nodular; not sharp-pointed.

Transverse section of leaf blade, physiology. C4; biochemical type NADP–ME (1 species); XyMS–. PCR cell chloroplasts centrifugal/peripheral. Mesophyll with radiate chlorenchyma. Leaf blade adaxially flat. Midrib conspicuous; having a conventional arc of bundles; with colourless mesophyll adaxially, or without colourless mesophyll adaxially. Bulliforms not present in discrete, regular adaxial groups (most of the epidermis being irregularly bulliform, apart from the midrib ‘hinge groups’). Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders present.

Cytology. Chromosome base number, x = 10. 2n = 20 and 40. Chromosomes ‘small’.

Classification. Watson & Dallwitz (1994): Panicoideae; Andropogonodae; Andropogoneae; Andropogoninae. Soreng et al. (2015): Panicoideae; Andropogonodae; Andropogoneae; Saccharinae. About 15 species.

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

Commonly adventive. Mesophytic; shade species; glycophytic.

Economic aspects. Significant weed species: M. vimineum.

Rusts and smuts. Rusts — Phakopsora and Puccinia. Taxonomically wide-ranging species: Phakopsora incompleta. Smuts from Ustilaginaceae. Ustilaginaceae — Sphacelotheca.

References, etc. Leaf anatomical: Metcalfe 1960; this project.

Illustrations. • M. vimineum: Hitchcock and Chase (1950). • General aspect (M.nudum): Gibbs Russell et al., 1990. • Abaxial epidermis of leaf blade (M. spectabile). • Leaf blade T.S., midrib region (M. spectabile)

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: 22nd January 2017.’.