| # The grass genera of the world |

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

### Hilaria Kunth

Named for Auguste St. Hilaire.

Including Hexarrhena Presl, Pleuraphis, Scleropelta Buckley, Symbasiandra Steud.

Habit, vegetative morphology. Perennial; rhizomatous, or stoloniferous, or caespitose. Culms 65–100 cm high; herbaceous; branched above, or unbranched above. Culm internodes solid. Leaves not basally aggregated; auriculate (e.g. H. jamesii), or non-auriculate. Leaf blades narrow; 1.5–5 mm wide; flat, or folded, or rolled; without abaxial multicellular glands; tending to pseudopetiolate, or not pseudopetiolate; without cross venation; persistent. Ligule a fringed membrane to a fringe of hairs. Contra-ligule absent.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets of sexually distinct forms on the same plant; hermaphrodite and male-only (in triplets, two staminate and one hermaphrodite); overtly heteromorphic. Apomictic, or reproducing sexually.

Inflorescence. Inflorescence a false spike, with spikelets on contracted axes; contracted; espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes very much reduced (to triplets of spikelets); disarticulating (though the main axis persists); falling entire (i.e., the triplets falling). Spikelets associated with bractiform involucres (constituted by the glumes of the three spikelets, which surround the triplet and are shed with it). The involucres shed with the fertile spikelets. Spikelets in triplets; not secund; sessile and subsessile (the central member subsessile, the others sessile).

Female-sterile spikelets. The lateral spikelets male, dorsally compressed, 2–3 flowered, their asymmetric glumes 4–5 nerved and awned at the side. Lodicules present. The male spikelets with glumes; 2–3 floreted.

Female-fertile spikelets. Spikelets 6–10 mm long; asymmetrically compressed, the floret being flattened laterally, and the glumes being displaced and flattened against the rachis; falling with the glumes (the clusters disarticulate). Rachilla terminated by a female-fertile floret.

Glumes two; more or less equal; shorter than the spikelets, or about equalling the spikelets; shorter than the adjacent lemmas, or long relative to the adjacent lemmas; joined, or free; displaced (against the rachis); hairy (ciliate); not pointed (cleft and ciliate); awned (dorsally from the mid-nerve, and in some species terminally from the laterals as well); carinate; similar. Lower glume 5–7 nerved. Upper glume 5–7 nerved. Spikelets with female-fertile florets only.

Female-fertile florets 1. Lemmas less firm than the glumes; not becoming indurated (membranous or hyaline); entire, or incised; when incised, 2 lobed; not deeply cleft; awnless, or mucronate, or awned. Awns when present, 1; median; dorsal; from near the top; non-geniculate; hairless; much shorter than the body of the lemma; entered by one vein. Lemmas carinate; without a germination flap; 3 nerved; with the nerves non-confluent. Palea present; relatively long; entire to apically notched; awnless, without apical setae; textured like the lemma (thin, hyaline); not indurated; 2-nerved; 2-keeled. Palea keels wingless. Lodicules absent (at least, in the material of H. jamesii seen). Stamens 3. Anthers 2.3–4 mm long (i.e. relatively long); not penicillate; without an apically prolonged connective. Ovary apically glabrous. Styles fused. Stigmas 2; red pigmented.

Fruit, embryo and seedling. Fruit ellipsoid. Hilum short. Embryo large; with an epiblast; with a scutellar tail; with an elongated mesocotyl internode. Embryonic leaf margins overlapping.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present; intercostal. Intercostal papillae over-arching the stomata; consisting of one symmetrical projection per cell, or consisting of one oblique swelling per cell (small, very thick walled, mostly towards one end of each long-cell and interstomatal). Long-cells of similar wall thickness costally and intercostally (quite thick walled). Mid-intercostal long-cells rectangular; having markedly sinuous walls (these pitted). Microhairs present; elongated; clearly two-celled; chloridoid-type (small, the basal cells relatively long, the distal cells very short). Microhair apical cell wall of similar thickness/rigidity to that of the basal cell. Microhairs 27–33 microns long. Microhair basal cells 24 microns long. Microhairs 7.5–10.5 microns wide at the septum. Microhair total length/width at septum 3–4.2. Microhair apical cells 7.5–10.5 microns long. Microhair apical cell/total length ratio 0.23–0.35. Stomata common; about 24 microns long. Subsidiaries dome-shaped and triangular. Guard-cells overlapped by the interstomatals (very slightly). Intercostal short-cells common; not paired (solitary, except that they are frequently paired with the bases of the abundant prickles). Intercostal silica bodies absent. Costal short-cells conspicuously in long rows. Costal silica bodies present in alternate cell files of the costal zones; ‘panicoid-type’; consistently dumb-bell shaped (large).

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

C4; biochemical type PCK (1 species); XyMS+. PCR sheath outlines even. PCR sheaths of the primary vascular bundles interrupted; interrupted abaxially only, or interrupted both abaxially and adaxially. PCR sheath extensions present. Maximum number of extension cells 1. PCR cell chloroplasts seemingly centripetal (in the poorish material seen). Mesophyll with radiate chlorenchyma; traversed by columns of colourless mesophyll cells (broad and conspicuous, between all the veins). Leaf blade ‘nodular’ in section; with the ribs more or less constant in size (broadly round topped). Midrib not readily distinguishable; with one bundle only. Bulliforms present in discrete, regular adaxial groups (in each groove); associated with colourless mesophyll cells to form deeply-penetrating fans (these linked with traversing columns of colourless cells). All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present (with all the bundles); forming ‘figures’ (anchors or I’s with all the bundles). Sclerenchyma all associated with vascular bundles. The lamina margins with fibres.

Cytology. Chromosome base number, x = 9. 2n = 36, 72, 86, 90, and 120. 4, 8, and 10 ploid (and higher). Nucleoli persistent.

Classification. Watson & Dallwitz (1994): Chloridoideae; main chloridoid assemblage. Soreng et al. (2015): Chloridoideae; Cynodonteae; Hilariinae. 9 species.

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

Xerophytic; species of open habitats. Arid and semi-arid plains.

Economic aspects. Important native pasture species: H. belangeri.

Rusts and smuts. Rusts — Puccinia. Taxonomically wide-ranging species: Puccinia aristidae. Smuts from Ustilaginaceae. Ustilaginaceae — Ustilago.

References, etc. Leaf anatomical: studied by us - H. jamesii Torr.

Special comments. Fruit data wanting.

Illustrations. • Hilaria belangeri: Hitchcock and Chase (1950).

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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