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The grass genera of the world

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

Exotheca Anderss.

Habit, vegetative morphology. Perennial; densely caespitose. Culms 30–200 cm high; herbaceous; branched above, or unbranched above. Culm nodes glabrous. Leaves not basally aggregated; auriculate (on the sheaths). Leaf blades linear; narrow; 2–4 mm wide; without cross venation. Ligule an unfringed membrane (adnate to the sheath auricles).

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets of sexually distinct forms on the same plant; hermaphrodite, male-only, and sterile; overtly heteromorphic (the imperfect spikelets awnless); in both homogamous and heterogamous combinations (with two proximal male or sterile, homogamous and homomorphic pairs and a terminal heterogamous triad).

Inflorescence. Inflorescence with long-pedunculate racemes of few spikelets, terminating culms on their distantly spaced branches; spatheate; a complex of ‘partial inflorescences’ and intervening foliar organs. Spikelet-bearing axes ‘racemes’ (short, of few spikelets); paired (the upper raceme base terete, 15–25 mm long so that the pair align end to end, by contrast with Hyparrhenia); disarticulating; disarticulating at the joints (disarticulating readily beneath the triad, persistent or tardily disarticulating between the basal homogamous pairs). ‘Articles’ somewhat auriculate beneath the terminal triad; disarticulating obliquely; glabrous. Spikelets in triplets (at the raceme tip), or paired (proximally); 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 solitary sessile spikelet of the triad), or male-only to sterile (the rest). The ‘longer’ spikelets male-only, or sterile.

Female-sterile spikelets. The lemmas awnless.

Female-fertile spikelets. Spikelets 12–16 mm long; not noticeably compressed (terete); falling with the glumes. Rachilla terminated by a female-fertile floret (the triad falling). Hairy callus present.

Glumes two; more or less equal; long relative to the adjacent lemmas; free; hairy; awnless; non-carinate; very dissimilar (both somewhat leathery, the G1 ending in a 2-keeled, bidentate, herbaceous beak, the G2 thinner, rounded on the back, 1-keeled at its membranous tip). Lower glume not two-keeled; convex on the back; not pitted; relatively smooth; 9 nerved. Upper glume 3 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. Spikelets with proximal incomplete florets. The proximal incomplete florets 1; epaleate; sterile. The proximal lemmas awnless; faintly 2–3 nerved; more or less equalling 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 stipitate, the teeth hyaline; less firm than the glumes; not becoming indurated; incised; 2 lobed; not deeply cleft (2-toothed); awned. Awns 1; median; from a sinus; geniculate; hairy (pilose); much longer than the body of the lemma. Lemmas lemmas ciliate; non-carinate; without a germination flap; 1 nerved. Palea present; conspicuous but relatively short; entire; awnless, without apical setae (but long-ciliate); not indurated (hyaline); nerveless; keel-less. Lodicules present; 2; free; glabrous. Stamens 3. Ovary apically glabrous. Stigmas 2.

Fruit, embryo and seedling. Fruit claviform-oblong.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells markedly different in shape costally and intercostally (the costals much narrower); of similar wall thickness costally and intercostally (thin walled). Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; elongated; clearly two-celled; panicoid-type; 54–60–66 microns long; 5.4–6–6.6 microns wide at the septum. Microhair total length/width at septum 8.6–9.9–11.7. Microhair apical cells 25.5–29–33 microns long. Microhair apical cell/total length ratio 0.45–0.48–0.53. Stomata common; 25.5–26.4–27 microns long. Subsidiaries predominantly high dome-shaped and triangular. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; not paired (mostly ostensibly solitary). Costal short-cells conspicuously in long rows. Costal silica bodies ‘panicoid-type’; predominantly short dumb-bell shaped.

Transverse section of leaf blade, physiology. C4; XyMS–. PCR sheath outlines even. PCR sheath extensions absent. Leaf blade adaxially flat. Midrib conspicuous; having a conventional arc of bundles (the large median flanked on either side by several smaller bundles, all displaced towards the abaxial surface); with colourless mesophyll adaxially. The lamina symmetrical on either side of the midrib. Bulliforms not present in discrete, regular adaxial groups (in that the epidermis is extensively bulliform). Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders present; forming ‘figures’ (the primaries with I’s and T’s). Sclerenchyma all associated with vascular bundles.

Phytochemistry. Leaves without flavonoid sulphates (1 species).

Classification. Watson & Dallwitz (1994): Panicoideae; Andropogonodae; Andropogoneae; Andropogoninae. Soreng et al. (2015): Panicoideae; Andropogonodae; Andropogoneae. 1 species.

Distribution, phytogeography, ecology. Tropical Africa.

Species of open habitats. Upland grassland.

Rusts and smuts. Rusts — Phakopsora and Puccinia. Taxonomically wide-ranging species: Phakopsora incompleta and ‘Uromycesclignyi.

References, etc. Morphological/taxonomic: Clayton 1966. Leaf anatomical: this project.

Special comments. Fruit data wanting. Illustrations. • Exotheca abyssinica: Hook. Ic. Pl. 31 (1922). • E. abyssinica: Jacques-Félix, 1962


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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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