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The grass genera of the world

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

Pseudarrhenatherum Rouy

~ Arrhenatherum

Including Thorea Rouy, Thoreochloa Holub

Habit, vegetative morphology. Perennial; caespitose. Culms 30–120(–150) cm high; herbaceous; unbranched above; not tuberous. Culm nodes hairy, or glabrous. Leaves non-auriculate. Leaf blades narrow; 0.6–4 mm wide (with prominent adaxial ribs, by contrast with Arrhenatherum); setaceous, or not setaceous; flat, or rolled (convolute); not pseudopetiolate; without cross venation; persistent. Ligule present; an unfringed membrane (to finely ciliate).

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

Inflorescence. Inflorescence paniculate; open (to rather dense); espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets not secund; pedicellate.

Female-fertile spikelets. Spikelets 5–10 mm long; compressed laterally; disarticulating above the glumes; not disarticulating between the florets. Rachilla prolonged beyond the uppermost female-fertile floret.

Glumes two; very unequal; long relative to the adjacent lemmas; pointed (acuminate); awnless; similar (thin). Lower glume 1 nerved, or 3 nerved. Upper glume 3 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets, or both distal and proximal to the female-fertile florets. The distal incomplete florets (when present) merely underdeveloped (a single rudiment). Spikelets with proximal incomplete florets. The proximal incomplete florets 1; paleate; male. The proximal lemmas awned (the awn geniculate, dorsal, from the middle or above); 5–9 nerved (?); similar in texture to the female-fertile lemmas (but apically bifid).

Female-fertile florets 1. Lemmas acute; decidedly firmer than the glumes; not becoming indurated; entire; pointed; awnless, or awned (?). Awns 1; median; dorsal; from near the top; non-geniculate. Lemmas non-carinate; without a germination flap; 5–9 nerved (?). Palea present; relatively long; awnless, without apical setae; not indurated; 2-nerved; 2-keeled. Palea keels wingless. Lodicules present; 2; free; membranous. Stamens 3. Anthers 2.5–5 mm long. Ovary apically hairy. Styles free to their bases. Stigmas 2.

Fruit, embryo and seedling. Fruit small (2.5–3 mm long); longitudinally grooved; compressed dorsiventrally. Hilum long-linear (but at 1/3 of the grain length, shorter than in Arrhenatherum). Embryo small.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Microhairs presumably absent (see Zarco 1985). Costal silica bodies not sharp-pointed.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with non-radiate chlorenchyma; without adaxial palisade. Leaf blade with distinct, prominent adaxial ribs (the ribs more prominent than in Arrhenatherum); with the ribs more or less constant in size (P. pallens), or with the ribs very irregular in sizes (P. longifolia). Midrib conspicuous to 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. All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; forming ‘figures’. Sclerenchyma all associated with vascular bundles (P. longifolium), or not all bundle-associated (P. pallens). The ‘extra’ sclerenchyma of P. pallens in a continuous abaxial layer.

Cytology. Chromosome base number, x = 7. 2n = 14 (+/- 1B). 2 ploid.

Classification. Watson & Dallwitz (1994): Pooideae; Poodae; Aveneae. Soreng et al. (2015): Pooideae; Poodae; Poeae; Aveninae. 2 species.

Distribution, phytogeography, ecology. Western Europe.

Species of open habitats. In dry grassland, calcicole.

References, etc. Morphological/taxonomic: Zarco 1985. Leaf anatomical: Zarco 1985.


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’.

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