| # 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 1–1.5 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. • Arundoclaytonia dissimilis: Davidse & Ellis, Ann. Miss. Bot. Gard. 74 (1987).

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: 15th June 2025. delta-intkey.com’.

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