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The grass genera of the world

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

Arundoclaytonia Davidse & Ellis

Named for W.D. Clayton, noted British agrostologist.

Habit, vegetative morphology. Perennial (of peculiar habit - see discussion by Davidse and Ellis 1987); caespitose (but the ‘basal’ tufts become raised up to 70 cm by the elongation of a perennial ‘trunk’). The flowering culms leafy. Culms 200–300 cm high (the vegetative culms many-noded, covered for the lower 2–70 cm to a thickness of 1.5–6cm by appressed aerial roots and the remnants of sheath bases); woody and persistent (towards the base); to 1.5 cm in diameter; sparsely branched above. Culm internodes becoming hollow (above). Young shoots intravaginal. Leaves mostly basal (the cauline leaves more or less reduced); spirally disposed (with 2/5 phyllotaxy). Leaf blades linear to linear-lanceolate; narrow to broad; those of the basal leaves 8–16 mm wide (and 45–80 cm long - those of the cauline leaves similar but smaller); flat (with involute margins and tips), or rolled (entirely involute); not pseudopetiolate; without cross venation; persistent. Ligule a fringed membrane; 0.3–0.9 mm long (the cilia 0.5–1.2 mm long). Contra-ligule absent.

Reproductive organization. Plants monoecious with all the fertile spikelets unisexual; without hermaphrodite florets. The spikelets of sexually distinct forms on the same plant (males and females in separate inflorescences, and reduced spikelets at the bases of the individual inflorescences); female-only, male-only, and sterile. The male and female-fertile spikelets in different inflorescences (the male inflorescences produced earlier than the females). The spikelets overtly heteromorphic.

Inflorescence. Inflorescence falsely paniculate (consisting of a false panicle of numerous pedunculate unisexual inflorescences); contracted (i.e. the individual inflorescences); capitate (each comprising a cluster of 7–20 spikelets: the male inflorescences usually longer-pedicellate, 9–13 mm wide and 15–23 mm high, the females 20–36 mm wide, 15–23 mm high); spatheate (each peduncle subtended by a sharp-pointed sheath). Spikelet-bearing axes persistent. Spikelets associated with bractiform involucres (the spikelet clusters each surrounded by one or two series of bracts and/or rudimentary spikelets). The involucres persistent on the rachis. Spikelets not secund; sessile to pedicellate (the pedicels to 0.5 mm long).

Female-sterile spikelets. Male spikelets 3.5–7.5 mm long, rounded on the back, disarticulating below the glumes, 3–9 flowered, the uppermost florets reduced; glumes 2, unequal, usually with cross-veinlets; lemmas shorter than paleas, 3–9 nerved; paleas two-keeled, lodicules absent, stamens 2, anthers basifixed and 2.2–2.9 mm long. The male spikelets with glumes; 3–9 floreted. The lemmas mucronate. Male florets 2 staminate.

Female-fertile spikelets. Spikelets 7–19 mm long; compressed laterally; falling with the glumes; not disarticulating between the florets; with conventional internode spacings. Rachilla prolonged beyond the uppermost female-fertile floret (the uppermost floret rudimentary); hairy; the rachilla extension with incomplete florets. Hairy callus present (under the glumes). Callus short.

Glumes two; very unequal; shorter than the spikelets; shorter than the adjacent lemmas; hairy (at the base, scaberulous above); pointed (broadly acute); awnless; non-carinate; similar (herbaceous, ovate). Lower glume about 0.6–0.75 times the length of the upper glume; 1–3 nerved. Upper glume 3–5 nerved. Spikelets with incomplete florets. The incomplete florets both distal and proximal to the female-fertile florets. The distal incomplete florets merely underdeveloped; awnless. The proximal incomplete florets 1; paleate, or epaleate. Palea of the proximal incomplete florets when present, reduced. The proximal incomplete florets sterile. The proximal lemmas awnless; 7–9 nerved; exceeded by the female-fertile lemmas; similar in texture to the female-fertile lemmas; not becoming indurated.

Female-fertile florets 1. Lemmas not becoming indurated; entire; pointed (ovate-acute); awnless; hairy (pilose at the base and between the nerves above, scaberulous elsewhere); apparently carinate; without a germination flap; 9–11 nerved (with conspicuous cross-veinlets); with the nerves non-confluent. Palea present; relatively long (much longer than the lemma, curved in the upper half); convolute around the flower; entire (pointed); awnless, without apical setae; spongy-thickened and smooth-shiny below, herbaceous above; not indurated; several nerved (9–13 nerved); shallowly grooved on the back. Lodicules absent. Stamens 0 (absent, or represented by an anterior pair of rudimentary staminodes). Ovary apically glabrous. Styles fused (into one). Stigmas 2 (inconspicuously plumose, exserted through the apical orifice of the convolute palea).

Fruit, embryo and seedling. Fruit free from both lemma and palea; medium sized (6–7 mm long); fusiform (narrowing above); not noticeably compressed (terete). Hilum short (elliptic-punctiform). Embryo large to small (0.3–0.4 times the length of the grain).

Abaxial leaf blade epidermis. Costal/intercostal zonation lacking. Papillae absent. Long-cells similar in shape costally and intercostally; of similar wall thickness costally and intercostally. Mid-intercostal long-cells rectangular; having markedly sinuous walls (these conspicuously pitted). Microhairs absent. Stomata common. Intercostal short-cells common; consistently in cork/silica-cell pairs; silicified. Costal short-cells predominantly paired. Costal silica bodies tall-and-narrow.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with non-radiate chlorenchyma; without adaxial palisade; not Isachne-type (the cells isodiametric in ts, tightly packed). Leaf blade with distinct, prominent adaxial ribs; with the ribs more or less constant in size (flat-topped). Midrib not readily distinguishable; with one bundle only. The lamina symmetrical on either side of the midrib. Bulliforms present in discrete, regular adaxial groups; in simple fans (a group in each furrow, each with an inflated median cell). All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; forming ‘figures’ (all the bundles with conspicuous I’s). Sclerenchyma not all bundle-associated (the abaxial girders linked to one another by a fibrous hypodermal layer). The ‘extra’ sclerenchyma in a continuous abaxial layer.

Special diagnostic feature. Plants not as in Steyermarkochloa (q.v.).

Classification. Watson & Dallwitz (1994): Arundinoideae (?); Steyermarkochloeae. Soreng et al. (2015): Panicoideae; Steyermarkochloeae. 1 species (A. dissimilis).

Distribution, phytogeography, ecology. South central Amazonian Brazil.

Xerophytic; species of open habitats; glycophytic.

References, etc. Morphological/taxonomic: Davidse and Ellis 1987. Leaf anatomical: Davidse and Ellis 1987.

Illustrations. • A. dissimilis: Davidse & Ellis, Ann. Miss. Bot. Gard. 74 (1987)

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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: 11th December 2017.’.