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The grass genera of the world

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

Aciachne Benth.

Habit, vegetative morphology. Perennial; cushion-forming and caespitose. Culms 1–2 cm high; herbaceous; unbranched above. Young shoots intravaginal. Leaves not basally aggregated; non-auriculate; without auricular setae. Sheath margins free. The sheaths overlapping, whitish, shining. Leaf blades linear; narrow; 0.3–0.8 mm wide; setaceous (recurved); acicular (and involute, pungent); without cross venation; persistent. Ligule an unfringed membrane; 1–1.5 mm long.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets of sexually distinct forms on the same plant, or all alike in sexuality; hermaphrodite, or hermaphrodite and sterile (the reduced inflorescence often exhibiting vestigial spikelets). Plants exposed-cleistogamous, or chasmogamous.

Inflorescence. Inflorescence reduced to a single spikelet, or few spikeleted; greatly reduced, barely extruded from the sheaths at anthesis; espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets not secund; pedicellate.

Female-fertile spikelets. Spikelets about 3 mm long; compressed laterally; disarticulating above the glumes. Rachilla terminated by a female-fertile floret. Hairy callus absent.

Glumes present; two; more or less equal; shorter than the spikelets; shorter than the adjacent lemmas; not pointed (obtuse); awnless; non-carinate (rounded on the back); similar. Lower glume 3 nerved, or 5 nerved. Upper glume 5 nerved. Spikelets with female-fertile florets only; without proximal incomplete florets.

Female-fertile florets 1. Lemmas fusiform; convolute; without a crown; similar in texture to the glumes (hard); becoming indurated (shiny); entire; pointed; awned. Awns 1; median; apical (the lemma extending into the sharp, firm point); non-geniculate; hairless; much shorter than the body of the lemma to about as long as the body of the lemma; persistent (with no line of demarcation from the body of the lemma). Lemmas hairless; non-carinate (abaxially rounded); having the margins lying flat on the palea; 3 nerved. Palea present; relatively long; tightly clasped by the lemma; not prow-tipped; awnless, without apical setae; textured like the lemma; 2-nerved; keel-less. Lodicules present; 3. Third lodicule present. Lodicules free; membranous (stipoid); glabrous; not toothed; heavily vascularized (the anterior members with one strong vein). Stamens 3. Anthers not penicillate; without an apically prolonged connective. Ovary apically glabrous. Styles free to their bases. Stigmas 2; white.

Fruit, embryo and seedling. Fruit small (1.3–1.6 mm long); ellipsoid; not noticeably compressed (terete). Hilum long-linear. Embryo small; not waisted. Endosperm hard; containing compound starch grains. Embryo with an epiblast; without a scutellar tail; with a negligible mesocotyl internode; with one scutellum bundle. Embryonic leaf margins meeting.

Abaxial leaf blade epidermis. Costal/intercostal zonation lacking. Papillae absent. Long-cells similar in shape costally and intercostally; of similar wall thickness costally and intercostally. Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs absent (and also absent adaxially). Stomata absent or very rare. Intercostal short-cells common; in cork/silica-cell pairs. Costal short-cells predominantly paired, or neither distinctly grouped into long rows nor predominantly paired. Costal silica bodies rounded, or ‘panicoid-type’.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with non-radiate chlorenchyma. Leaf blade adaxially flat. Midrib conspicuous; with one bundle only. Bulliforms present in discrete, regular adaxial groups, or not present in discrete, regular adaxial groups; in simple fans (or these ill defined). All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders absent. Sclerenchyma not all bundle-associated. The ‘extra’ sclerenchyma in a continuous abaxial layer.

Cytology. Chromosome base number, x = 11. 2n = 22. 2 ploid.

Classification. Watson & Dallwitz (1994): Stipoideae; Stipeae. Soreng et al. (2015): Pooideae; Stipeae. 3 species.

Distribution, phytogeography, ecology. High Andes, northern Argentina to Costa Rica.

Xerophytic; species of open habitats; glycophytic. High altitude grassland.

References, etc. Morphological/taxonomic: Macfarlane and Watson 1980; Laegaard 1987. Leaf anatomical: this project.

Illustrations. • A. pulvinata: Hook. Icones Plantarum 14 (1881). • A. pulvinata: 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: 13th November 2017. delta-intkey.com/grass’.

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