| # The grass genera of the world |

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

### Erioneuron Nash

Habit, vegetative morphology. Perennial; densely caespitose. Culms 15–40 cm high; herbaceous. Leaves mostly basal; non-auriculate. Leaf blades narrow; 1–1.5 mm wide; flat, or folded, or rolled; exhibiting multicellular glands abaxially (at the base of macrohairs). The abaxial leaf blade glands intercostal. Leaf blades without cross venation. Ligule a fringe of hairs.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets.

Inflorescence. Inflorescence few spikeleted; paniculate; contracted; capitate, or more or less ovoid. Inflorescence with axes ending in spikelets. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets not secund; subsessile to pedicellate.

Female-fertile spikelets. Spikelets 5.5–20 mm long; compressed laterally (plump); disarticulating above the glumes; disarticulating between the florets; with conventional internode spacings. Rachilla prolonged beyond the uppermost female-fertile floret; the rachilla extension with incomplete florets.

Glumes two; relatively large; very unequal to more or less equal; shorter than the spikelets to about equalling the spikelets; shorter than the adjacent lemmas to long relative to the adjacent lemmas; free; hairless; glabrous; pointed; awned to awnless; similar (membranous to leathery). Lower glume 1 nerved. Upper glume 1 nerved. Spikelets with incomplete florets. The incomplete florets distal to the female-fertile florets. The distal incomplete florets merely underdeveloped; awned. Spikelets without proximal incomplete florets.

Female-fertile florets 4–18. Lemmas not becoming indurated; entire, or incised; when incised, 2 lobed; not deeply cleft; awned. Awns 1 (often accompanied by two lateral mucros); median; from a sinus (or from between the lobes); non-geniculate; hairless; much shorter than the body of the lemma to about as long as the body of the lemma; entered by one vein. Lemmas hairy (on the nerves, at least below); non-carinate (dorsally rounded); without a germination flap; 3 nerved (the laterals marginal). Palea present (pilose below); conspicuous but relatively short; entire; awnless, without apical setae; 2-nerved; 2-keeled. Palea back hairy (villous below). Palea keels hairy (ciliolate to ciliate). Lodicules present; 2; sometimes joined to the palea; fleshy; glabrous. Stamens 1–3. Anthers 0.5–1.2 mm long. Ovary apically glabrous. Stigmas 2; white.

Fruit, embryo and seedling. Fruit small (about 1.6 mm long); ellipsoid; compressed laterally; smooth (translucent). Hilum short. Pericarp fused. Embryo large (projecting downwards); waisted. Endosperm hard. Embryo with an epiblast; with a scutellar tail; with an elongated mesocotyl internode. Embryonic leaf margins meeting.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells of similar wall thickness costally and intercostally (fairly thick walled). Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; more or less spherical to elongated; clearly two-celled; chloridoid-type (large, the apical cells massive, bulbous, thick walled). Microhair apical cell wall of similar thickness/rigidity to that of the basal cell. Microhairs (25.5–)28.5–34.5(–39) microns long. Microhair basal cells 15 microns long. Microhairs 12–15 microns wide at the septum. Microhair total length/width at septum 2.1–2.6. Microhair apical cells 15–22.5 microns long. Microhair apical cell/total length ratio 0.5–0.75. Stomata common; 24–28.5 microns long. Subsidiaries predominantly low to high dome-shaped. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells absent or very rare. Intercostal silica bodies absent. Costal short-cells conspicuously in long rows, or predominantly paired. Costal silica bodies present throughout the costal zones, or confined to the central file(s) of the costal zones; rounded to saddle shaped (predominating over the midrib), or tall-and-narrow (a few, over minor veins), or ‘panicoid-type’ (predominating over the minor veins); when panicoid type, short dumb-bell shaped, or cross shaped.

Transverse section of leaf blade, physiology. Lamina mid-zone in transverse section open.

C4; XyMS+. PCR sheath outlines even. PCR sheaths of the primary vascular bundles complete. PCR sheath extensions absent. PCR cell chloroplasts centripetal. Mesophyll with radiate chlorenchyma; traversed by columns of colourless mesophyll cells. Leaf blade ‘nodular’ in section to adaxially flat. Midrib conspicuous (via the rounded keel and a large abaxial sclerenchyma group, and lack of an adaxial strand); with one bundle only. Bulliforms present in discrete, regular adaxial groups; associated with colourless mesophyll cells to form deeply-penetrating fans (these incorporated in the traversing colourless columns). Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders absent (adaxial strands only, and abaxial strands or girders). Sclerenchyma all associated with vascular bundles. The lamina margins with fibres (large).

Cytology. Chromosome base number, x = 8. 2n = 16. 2 ploid.

Classification. Watson & Dallwitz (1994): Chloridoideae; main chloridoid assemblage. Soreng et al. (2015): Chloridoideae; Cynodonteae; Scleropogoninae. 5 species.

Distribution, phytogeography, ecology. Southwest U.S.A. and Mexico.

Species of open habitats. Rocky slopes.

References, etc. Leaf anatomical: studied by us - E.n pilosum.

Special comments. Formerly a segregate of Tridens.

Illustrations. • Erioceuron pulchellum (as Tridens): Hitchcock and Chase (1950). • Erioneuron pilosum: Nicora & Rúgolo de Agrasar (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’.

Contents