DELTA home

The grass genera of the world

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

Oryzopsis Michx.

From Oryza (q.v., rice) and opsis (appearance), alluding to a fancied similarity.

Including Dilepyrum Raf., Piptatheropsis p.p.

Excluding Piptatherum

Habit, vegetative morphology. Perennial; caespitose. Culms 10–150 cm high; unbranched above. Culm nodes glabrous. Culm leaves present. Culm internodes solid, or hollow. Young shoots extravaginal, or intravaginal. Leaves non-auriculate. Leaf blades greatly reduced (on upper culms, sometimes), or not all greatly reduced; broad, or narrow; 0.7–15 mm wide; flat, or folded, or rolled (involute); not pseudopetiolate; without cross venation; persistent; rolled in bud, or once-folded in bud. Ligule an unfringed membrane, or a fringed membrane (?); truncate to not truncate; 0.2–15 mm long.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets; exposed-cleistogamous, or chasmogamous.

Inflorescence. Inflorescence paniculate; open; with capillary branchlets, or without capillary branchlets; espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets not secund; pedicellate.

Female-fertile spikelets. Spikelets 2–10 mm long; compressed laterally to not noticeably compressed; disarticulating above the glumes. Rachilla terminated by a female-fertile floret. Hairy callus present. Callus short; blunt.

Glumes two; more or less equal; about equalling the spikelets, or exceeding the spikelets; long relative to the adjacent lemmas; pointed; awnless; non-carinate; similar (membranous). Lower glume 3–7 nerved. Upper glume 3–7 nerved. Spikelets with female-fertile florets only; without proximal incomplete florets.

Female-fertile florets 1. Lemmas convolute (concealing the palea); not saccate; without a crown; decidedly firmer than the glumes; becoming indurated; entire, or incised (?); if incised, obscurely 2 lobed; not deeply cleft; awned. Awns 1; median; from a sinus, or apical (?); geniculate; hairless, or hairy; much shorter than the body of the lemma to much longer than the body of the lemma; deciduous, or persistent. Lemmas hairy, or hairless; non-carinate; having the margins lying flat on the palea; 3–5(–9) nerved. Palea present (completely covered by the lemma); relatively long (equalling the lemma); prow-tipped; awnless, without apical setae; textured like the lemma (leathery); not indurated; 2-nerved; keel-less. Palea back glabrous (?). Lodicules present; 2; free; membranous (stipoid); ciliate, or glabrous (?); not toothed; heavily vascularized. Stamens 3. Anthers 1.5–5 mm long; penicillate (usually), or not penicillate. Ovary apically glabrous. Styles fused. Stigmas 2.

Fruit, embryo and seedling. Fruit free from both lemma and palea; small to medium sized (1.5–4.0 mm long). Hilum long-linear. Embryo large to small (1/4 to 1/2 the grain length). Endosperm hard; without lipid. Embryo with an epiblast (the epiblast usually short and truncate, sometimes long and notched in O. canadensis); without a scutellar tail (with a 95–130 degree angle between coleoptile and coleorhiza); with a negligible mesocotyl internode. Embryonic leaf margins meeting.

Seedling with a long mesocotyl. First seedling leaf with a well-developed lamina. The lamina narrow; erect.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous (in O. asperifolia). Papillae absent. Long-cells similar in shape costally and intercostally (narrow-rectangular); of similar wall thickness costally and intercostally (thick-walled, the costals somewhat less so). Mid-intercostal long-cells rectangular; having markedly sinuous walls (pitted). Microhairs absent. Stomata common; 21–24 microns long. Subsidiaries low to high dome-shaped. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; in cork/silica-cell pairs; silicified. Intercostal silica bodies present and perfectly developed; rounded (elliptical or oval), or crescentic (slightly), or tall-and-narrow. Prickles present. Costal short-cells predominantly paired (and a few short rows, in O. asperifolia). Costal silica bodies present and well developed; rounded (including potato shapes), or tall-and-narrow (to more or less cubical), or crescentic.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with radiate chlorenchyma to with non-radiate chlorenchyma; with adaxial palisade to without adaxial palisade. Leaf blade with distinct, prominent adaxial ribs, or ‘nodular’ in section; with the ribs very irregular in sizes. Midrib not readily distinguishable; with one bundle only. Bulliforms present in discrete, regular adaxial groups (in the furrows); in simple fans. All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; forming ‘figures’. Sclerenchyma all associated with vascular bundles.

Culm anatomy. Culm internode bundles in one or two rings.

Phytochemistry. Tissues of the culm bases with abundant starch.

Cytology. Chromosome base number, x = 11, 12, and 14; 2, 4, 5, and 10 ploid, or 11 ploid.

Classification. Watson & Dallwitz (1994): Stipoideae; Stipeae. Soreng et al. (2015): Pooideae; Stipeae. About 4 species.

Distribution, phytogeography, ecology. North America.

Commonly adventive. Mesophytic to xerophytic; shade species, or species of open habitats (mainly).

Rusts and smuts. Rusts — Puccinia. Smuts from Tilletiaceae and from Ustilaginaceae. Tilletiaceae — Urocystis. Ustilaginaceae — Ustilago.

References, etc. Morphological/taxonomic: Freitag 1975. Leaf anatomical: Metcalfe 1960 (for O. asperifolia only.

Special comments. The North American species usually known as Oryzopsis micrantha and O. racemosa are referred here to the otherwise Old World genus Piptatherum, and O. hymenoides to Achnatherum, cf. Barkworth 1993. This at least permits Oryzopsis sensu stricto to be distinguished from Piptatherum on morphological grounds, but negates the former geographical distinction. While Oryzopsis sensu lato (i.e. including Piptatherum) is clearly unsatisfactory, the present solution is probably transient. Illustrations. • O. holciformis: Fl. Iraq, 1968


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: 13th November 2017. delta-intkey.com/grass’.

Contents