| # The grass genera of the world |

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

### Achnatherum Beauv.

~ Stipa

Type species: Lecto: A. calamagrostis (L.) P.Beauv.

Including Aristella Bertol., Eriocoma Nutt., Fendleria Steud., Lasiagrostis Link, Macrochloa Kunth, Orthoraphium Nees, (?)Patis Ohwi, Timouria Roshev., Trichosantha Steud.

Excluding Jarava, Trikeraia

Habit, vegetative morphology. Annual (usually), or perennial (rarely); caespitose (sometimes shortly rhizomatous). Culms 15–150(–250) cm high; herbaceous; unbranched above (simple). Culm nodes hairy, or glabrous (?). Culm internodes solid (rarely), or hollow (?). Young shoots extravaginal, or intravaginal. Leaves mostly basal, or not basally aggregated; auriculate, or non-auriculate (?). Leaf blades linear; narrow; 1–7 mm wide; setaceous, or not setaceous (?); flat (e.g. sect. Lasiagrostis), or folded, or rolled (then involute or convolute), or acicular; not pseudopetiolate; without cross venation; persistent; rolled in bud, or once-folded in bud (?). Ligule an unfringed membrane, or a fringed membrane (usually leathery-membranous); 0.2–7(–10) mm long. Contra-ligule present, or absent.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets hermaphrodite. Plants inbreeding; exposed-cleistogamous, or chasmogamous (judging by the recorded anther lengths); with hidden cleistogenes, or without hidden cleistogenes (?). The hidden cleistogenes (if present) in the leaf sheaths.

Inflorescence. Inflorescence few spikeleted to many spikeleted; paniculate; not deciduous; open, or contracted; with capillary branchlets, or without capillary branchlets (?); espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets not secund; pedicellate (the pedicels scabridulous or scabrous).

Female-fertile spikelets. Spikelets 5.5–14(–23) mm long (usually less than 15 mm, but 14–23 mm in A. capensis, and with other members of sect. Stipella also falls in the higher part of the range); fusiform, or subcylindrical, or subspherical, or suborbicular; compressed laterally to not noticeably compressed; disarticulating above the glumes. Rachilla terminated by a female-fertile floret. Hairy callus present. The callus hairs white. Callus short (mostly 0.2–1, but to 2.5 mm long in A. capensis); pointed to blunt.

Glumes two; very unequal to more or less equal (the lower often shorter); (the upper) about equalling the spikelets, or exceeding the spikelets; long relative to the adjacent lemmas; pointed (acute or acuminate), or not pointed; awned, or awnless (sometimes aristate?); carinate, or non-carinate (?); similar (leathery-membranous, usually lanceolate, the lower sometimes ovate). Lower glume 1 nerved, or 3 nerved, or 5 nerved. Upper glume 3 nerved, or 5 nerved, or 7 nerved. Spikelets with female-fertile florets only; without proximal incomplete florets.

Female-fertile florets 1. Lemmas not thickened apically; partially convolute (not totally concealing the palea), or not convolute; not saccate; without a crown; similar in texture to the glumes, or decidedly firmer than the glumes; usually striate (with round to oval silica bodies in the abaxial epidermis); becoming indurated (rarely), or not becoming indurated (mostly stiffly membranous to leathery); entire (sect. Achnatheropsis), or incised; usually 2 lobed; not deeply cleft (the lobes usually membranous, 0.5–2 mm long, occasionally thick or awnlike); awned. Awns 1 (usually), or 3 (when the lobes aristate); median, or median and lateral; the median different in form from the laterals (when laterals present); from a sinus; from near the top; non-geniculate (when short), or geniculate (when more than 1 cm long); hairless to hairy (this being a cited ‘difference’ from Ptilagrostis), or hairy to long-plumose (rarely, e.g. A. duthiei, A. haussknechtii); much shorter than the body of the lemma to much longer than the body of the lemma (3–80 mm long); entered by one vein, or entered by several veins (?); deciduous (when short and non-geniculate), or persistent. Awn bases twisted, or not twisted (e.g. sect. Aristella). Lemmas hairy (usually pubescent, the hairs usually evenly distributed), or hairless (occasionally glabrous). The hairs not in transverse rows. Lemmas non-carinate (terete); having the margins lying flat on the palea; without a germination flap; 3–7 nerved (?). Palea present; relatively long to conspicuous but relatively short (from about a third of the lemma length to equalling it); tightly clasped by the lemma; not prow-tipped (flat); entire (the apex rounded), or apically notched (A. stillmanii); awnless, without apical setae (usually), or with apical setae (A. stillmannii - the veins extended); textured like the lemma; not indurated; 2-nerved (the veins not extending to the apex); keel-less (flat). Palea back glabrous to hairy. Lodicules present; 3 (but the posterior member sometimes very small, e.g. in sect. Stipella). Third lodicule present. Lodicules free; membranous (‘stipoid’); ciliate, or glabrous; not toothed; heavily vascularized, or not or scarcely vascularized (?). Stamens 3. Anthers 1–9 mm long; penicillate, or not penicillate. Ovary apically glabrous. Styles free to their bases; free. Stigmas 2; white.

Fruit, embryo and seedling. Fruit free from both lemma and palea; small to medium sized (2–6 mm long); linear to fusiform; not grooved; compressed laterally. Hilum long-linear. Embryo small; not waisted. Endosperm hard; without lipid; containing only simple starch grains, or containing compound starch grains (?). Embryo with an epiblast (the epiblast variously long or short, and acute, truncate or notched); 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 short mesocotyl, or with a long mesocotyl (?). First seedling leaf with a well-developed lamina. The lamina narrow; erect; 3–5 veined (?).

Ovule, embryology. Micropyle oblique. Outer integument extensive, being absent only from the micropylar region; two cells thick at the micropylar margin. Inner integument continuous, the micropyle constricted; thickened around the micropyle. Synergids not haustorial (Stipa elmeri); without large, globular starch grains.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells similar in shape costally and intercostally to markedly different in shape costally and intercostally, or markedly different in shape costally and intercostally (the costals often much narrower); of similar wall thickness costally and intercostally to differing markedly in wall thickness costally and intercostally (thin to fairly thick walled, those of the costals usually somewhat thicker). Mid-intercostal long-cells rectangular (in most species recorded), or rectangular to fusiform; having markedly sinuous walls (A. giganteum, A. splendens, with fairly thick, pitted walls), or having straight or only gently undulating walls (commonly, thin-walled and not conspicuously pitted). Microhairs absent (possibly present adaxially - cf. other stipoids). Stomata common (fairly infrequent (e.g. A. calamagrostis) to abundant), or absent or very rare (A. columbianum); markedly variable in length from species to species. Subsidiaries non-papillate; low to high dome-shaped (mostly), or parallel-sided to dome-shaped (A. filifolium). Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common (in most species recorded), or absent or very rare (e.g. uncommon in A. filifolium); in cork/silica-cell pairs, or in cork/silica-cell pairs and not paired (sometimes some solitary); silicified (usually), or not silicified (in A. filifolium, except those bordering the veins). Intercostal silica bodies present and perfectly developed; mostly rounded and crescentic (small, a few somewhat vertically elongated in A. columbianum and A. filifolium), or cross-shaped (irregularly), or tall-and-narrow and vertically elongated-nodular (especially in A. giganteum). No macrohairs or prickles seen in A. columbianum, A. giganteum, a few large costal prickles in A. filifolium. Costal short-cells conspicuously in long rows (at least over the large veins, but the rows sometimes broken up by fairly long to very long (e.g. A. filifolium) intervening short-cells, and some pairs also commonly present), or predominantly paired (A. giganteum). Costal silica bodies present and well developed; present throughout the costal zones; horizontally-elongated smooth (e.g. A. splendens, or oblong to cuboid, e.g. A. calamagrostis), or horizontally-elongated smooth and ‘panicoid-type’ (e.g. in A. dregeana), or horizontally-elongated crenate/sinuous, horizontally-elongated smooth, rounded, and ‘panicoid-type’ (with intermediates, e.g. A. filifolium), or crescentic (large and predominating only in A. giganteum of the species recorded); when panicoid type, cross shaped, or dumb-bell shaped, or cross shaped and dumb-bell shaped, or dumb-bell shaped and nodular (e.g. A. columbianum exhibits mixtures of small, irregular dumb-bells and nodular forms, some of the latter approaching the crenate (pooid) type, while irregular, slightly nodular forms predominate in A. filifolium).

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll somewhat with radiate chlorenchyma (e.g. A. capensis), or with non-radiate chlorenchyma (the chlorenchyma usually close packed); without adaxial palisade. Leaf blade with distinct, prominent adaxial ribs; with the ribs more or less constant in size (e.g. A. filifolium, A. giganteum, with no ribs over the smaller bundles), or with the ribs very irregular in sizes (alternating large and small, e.g. A. columbianum). Midrib conspicuous (e.g. A. calamagrostis, A. dregeanum, A. filifolium), or not readily distinguishable (e.g. A. columbianum, A. giganteum, A. splendens); with one bundle only (usually), or having a conventional arc of bundles (e.g. A. dregeanum, and dubiously in A. calamagrostis); without colourless mesophyll adaxially. The lamina symmetrical on either side of the midrib. Bulliforms present in discrete, regular adaxial groups (usually, in the furrows), or not present in discrete, regular adaxial groups (e.g. A. giganteum seemingly lacking bulliforms); in simple fans (these from small and ill defined, e.g. A. columbianum, to well developed, e.g. A filifolium). All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present (often with only the main bundles, the smaller ones then having abaxial strands or girders only); forming ‘figures’ (at least the large bundles usually with conspicuous ‘anchors’, I’s or T’s). Sclerenchyma all associated with vascular bundles. The lamina margins with fibres.

Phytochemistry. Leaves without flavonoid sulphates.

Cytology. Chromosome base number, x = 11 and 12. 2n = 24, or 18, or 26, or 28, or 34, or 36, or 42, or 48. 2 ploid, or 3 ploid, or 4 ploid. Chromosomes ‘medium sized’. Nucleoli disappearing before metaphase.

Classification. Watson & Dallwitz (1994): Stipoideae; Stipeae. Soreng et al. (2015): Pooideae; Stipeae. About 100 species (?).

Distribution, phytogeography, ecology. Eurasia, Africa, North and South America, New Zealand.

Commonly adventive. Mesophytic to xerophytic; species of open habitats.

Hybrids. Intergeneric hybrids with Nassella - ×Achnella Barkworth, ×Stiporyzopsis Johnson and Rogler.

Rusts and smuts. Rusts — Puccinia. Smuts from Tilletiaceae and from Ustilaginaceae (?).

References, etc. Morphological/taxonomic: Tsvelev 1976, Barkworth and Everett 1987, Barkworth 1993. Leaf anatomical: Metcalfe 1960 - A. calamagrostis, A. dregeana, A. splendens; studied by us - A. columbiana, A. filifolia Nees, A. gigantea Link.

Special comments. This is an unsatisfactory description of a Stipa segregate. See further comment under Stipa sensu lato.

Illustrations. • Achnatherum calamagrostis: P. Beauv. (1812). • A. calamagrostis, with representatives of other genera: Beauv., plate VI (1812). • Beauv., plate VI legend. • A. calamagrostis: Voekler (2009), Wikimedia Commons.

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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