| # The grass genera of the world |

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

### Arberella Soderstrom & Calderón

Named for Agnes Chase, eminent British plant morphologist.

Habit, vegetative morphology. Perennial; caespitose. The flowering culms leafy (i.e. the culms not dimorphic). Culms 25–35 cm high; herbaceous; unbranched above. Culm internodes solid. Plants unarmed. Leaves not basally aggregated; with auricular setae (but these tiny). Leaf blades linear-lanceolate to ovate-lanceolate; broad, or narrow; 5–30 mm wide (from 5–14 cm long, asymmetric at base); flat; pseudopetiolate; without cross venation; rolled in bud. Ligule an unfringed membrane (ciliolate), or a fringed membrane (ciliate); 0.4–0.8 mm long.

Reproductive organization. Plants monoecious with all the fertile spikelets unisexual; without hermaphrodite florets. The spikelets of sexually distinct forms on the same plant; female-only and male-only. The male and female-fertile spikelets segregated, in different parts of the same inflorescence branch (terminal female spikelets, smaller male ones below). The spikelets overtly heteromorphic.

Inflorescence. Inflorescence determinate, or indeterminate (according to interpretation); paniculate (several panicles (synflorescences) from each of several to many nodes); with capillary branchlets. Inflorescence (female) with axes ending in spikelets. Inflorescence spatheate (prophylla projecting beyond the axillant sheaths); a complex of ‘partial inflorescences’ and intervening foliar organs. Spikelet-bearing axes paniculate; persistent. Spikelets solitary; not secund; pedicellate (the pedicels of female spikelets thickened at the tip).

Female-sterile spikelets. Male spikelets mainly paired, without glumes; 3.2–6.4 mm long, with one floret, lemma and palea membranous; lodicules 3, fleshy; stamens 3. The male spikelets without glumes; 1 floreted. The lemmas awnless. Male florets 3 staminate.

Female-fertile spikelets. Spikelets 8.5–22 mm long; compressed dorsiventrally; disarticulating above the glumes; with conventional internode spacings, or with a distinctly elongated rachilla internode above the glumes. Rachilla terminated by a female-fertile floret. Hairy callus absent. Callus absent, or short (columnar).

Glumes two; relatively large; very unequal, or more or less equal; about equalling the spikelets to exceeding the spikelets; long relative to the adjacent lemmas; hairless; glabrous; pointed; awnless; non-carinate; similar (membranous, ovate-lanceolate, attenuate). Lower glume 5–6 nerved, or 9–11 nerved (with transverse veinlets). Upper glume 5–9 nerved (with transverse veinlets). Spikelets with female-fertile florets only.

Female-fertile florets 1. Lemmas completely embracing the palea, ovate lanceolate; decidedly firmer than the glumes; becoming indurated (fleshy early, hardening later); entire; pointed; awnless (but apiculate); hairy (pilose basally and along margins); non-carinate; having the margins inrolled against the palea; with a clear germination flap (apparently); 5 nerved. Palea present; relatively long; entire; awnless, without apical setae; indurated (textured like the lemma); several nerved (4–10); keel-less. Lodicules present; 3; free; fleshy; glabrous; heavily vascularized. Stamens 0. Ovary apically glabrous. Styles fused (into one long ribbon-like style). Stigmas 2 (plumose).

Fruit, embryo and seedling. Fruit subglobose (ovoid-spherical); not noticeably compressed. Hilum long-linear. Pericarp fused.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous (the costal zones narrow and distant from one another, the wide intercostal zones themselves subdivided into a broad, non-papillate, astomatal median zone, and narrow, stomatal, densely papillate zones bordering each costal zone). Papillae present; costal and intercostal (completely lacking from the wide median regions of the intercostal zones and near the blade margins, but abundant elsewhere). Intercostal papillae over-arching the stomata (except near the blade margins); several per cell (mostly of the coronate-pit type (cf. Poa helmsii) or branched, large, in one or more irregular longitudinal rows per cell). Long-cells of similar wall thickness costally and intercostally. Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present (abundant near the blade margins, seemingly absent elsewhere); elongated; clearly two-celled; panicoid-type; 45–60 microns long; 5.4–6.6 microns wide at the septum. Microhair total length/width at septum 5.4–6.8. Microhair apical cells (21–)30(–37.5) microns long. Microhair apical cell/total length ratio 0.5–0.53. Stomata common (bordering the costae, but sunken, overarched and hard to find except near the blade margins, where they are completely exposed); 24–27 microns long. Intercostal short-cells common; in cork/silica-cell pairs; silicified. Intercostal silica bodies vertically elongated-nodular. Large prickles without basal rosettes common costally and intercostally. Crown cells absent. Costal short-cells conspicuously in long rows. Costal silica bodies oryzoid (some, being vertically elongated crosses), or ‘panicoid-type’; abundant, warty, mostly cross shaped.

Transverse section of leaf blade, physiology. C3. Mesophyll seemingly without arm cells; with fusoids. The fusoids external to the PBS. Leaf blade with low, very wide adaxial ribs. Midrib conspicuous; having a conventional arc of bundles (one large, median primary bundle and a minor bundle on either side); with colourless mesophyll adaxially. The lamina symmetrical on either side of the midrib. Bulliforms not present in discrete, regular adaxial groups (the adaxial epidermis extensively bulliform, with the cells gradually increasing in size towards the middle of each intercostal region). All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; forming ‘figures’ (all the bundles with slender girders, some of these constituting slight I’s or ‘anchors’). Sclerenchyma all associated with vascular bundles.

Cytology. Chromosome base number, x = 11. 2n = 22. 2 ploid.

Classification. Watson & Dallwitz (1994): Bambusoideae; Oryzodae; Olyreae. Soreng et al. (2015): Bambusoideae; Bambusodae; Olyreae; Olyrinae. 3 species.

Distribution, phytogeography, ecology. Tropical America.

References, etc. Morphological/taxonomic: Soderstrom and Calderón 1979b; Soderstrom and Zuloaga 1988. Leaf anatomical: studied by us - A. dressleri Soderstrom & Calderon.

Illustrations. • Arberella costaricensis, as Raddia: Pohl (1980), Fl. Costaricensis. • Arberella dressleri, abaxial epidermis of leaf blade: this project. • Abaxial leaf blade epidermal silica bodies of Arberella dressleri: this project. • Arberella dressleri, TS leaf blade: this project.

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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