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From the Greek akra (the terminal point) and achne (a scale), referring to the fine-pointed glumes and lemmas.
Type species: Type: A. verticillata (Roxb.) Chiov. = A. racemosa (B.Heyne ex Roem. & Schult.) Ohwi.
Including Arthrochloa Lorch, Camusia Lorch, Normanboria Butzin
Habit, vegetative morphology. Annual; caespitose. Culms 12–80 cm high; herbaceous. Culm nodes glabrous. Culm internodes solid. Leaves not basally aggregated; non-auriculate. Leaf blades broadly linear (tapered to a hairlike tip); broad to narrow; flat; without abaxial multicellular glands; without cross venation; persistent. Ligule a fringed membrane to a fringe of hairs.
Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets; exposed-cleistogamous, or chasmogamous.
Inflorescence. Inflorescence of spicate main branches; subdigitate (usually with the lower spikes scattered, but becoming subdigitate above), or non-digitate (A. racemosa). Primary inflorescence branches 4–20. Rachides flattened. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. The racemes spikelet bearing to the base. Spikelet-bearing axes with substantial rachides; persistent. Spikelets solitary; secund (on the dorsiventral, slender, flattened rachis); biseriate; subsessile.
Female-fertile spikelets. Spikelets 5.5–13 mm long; adaxial; compressed laterally; disarticulating above the glumes, or falling with the glumes, or not disarticulating (the lemmas falling acropetally from the rachilla, but the spikelet often falling wholly or in part before all the lemmas have been shed); not disarticulating between the florets to disarticulating between the florets (the rachilla tough or breaking irregularly, the paleas persistent). Rachilla prolonged beyond the uppermost female-fertile floret; hairless; the rachilla extension with incomplete florets. Hairy callus absent. Callus absent.
Glumes two; relatively large; more or less equal; shorter than the spikelets; shorter than the adjacent lemmas; dorsiventral to the rachis; awnless (but subulate via an excurrent mid-nerve), or awned (G2); carinate; similar (thinly cartilaginous). Lower glume shorter than the lowest lemma; 1 nerved. Upper glume 1 nerved. Spikelets with incomplete florets. The incomplete florets distal to the female-fertile florets.
Female-fertile florets (6–)8–20. Lemmas similar in texture to the glumes to decidedly firmer than the glumes (cartilaginous); not becoming indurated; incised; not deeply cleft (slightly notched via the slightly excurrent lateral nerves); mucronate (from the midnerve); hairless; glabrous; carinate; 3 nerved (the laterals closer to the margins than to the mid-nerve, and excurrent as small teeth). Palea present (lanceolate); 2-nerved. Lodicules present; 2; free; fleshy; glabrous. Stamens 3. Anthers not penicillate. Ovary apically glabrous. Styles free to their bases. Stigmas 2.
Fruit, embryo and seedling. Fruit free from both lemma and palea; small (0.8–1.1 mm long); ellipsoid; deeply longitudinally grooved (on the hilar side); compressed dorsiventrally; sculptured. Hilum short. Pericarp free. Embryo large; not waisted. Endosperm containing compound starch grains. Embryo with an epiblast; with a scutellar tail; with an elongated mesocotyl internode.
Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. 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 present; chloridoid-type. Microhair apical cell wall of similar thickness/rigidity to that of the basal cell. Microhair basal cells 6–9 microns long. Stomata common. Subsidiaries dome-shaped (usually), or triangular (rarely). Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells absent or very rare. Intercostal silica bodies absent. Costal short-cells conspicuously in long rows. Costal silica bodies present in alternate cell files of the costal zones; saddle shaped, or ‘panicoid-type’ (rarely).
Transverse section of leaf blade, physiology. Lamina mid-zone in transverse section open.
C4; XyMS+. PCR sheath outlines even. PCR sheaths of the primary vascular bundles complete. PCR cell chloroplasts centripetal. Mesophyll with radiate chlorenchyma. Leaf blade ‘nodular’ in section. Midrib conspicuous; having a conventional arc of bundles; with colourless mesophyll adaxially. All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present (with the major bundles); forming ‘figures’, or nowhere forming ‘figures’. Sclerenchyma all associated with vascular bundles.
Classification. Watson & Dallwitz (1994): Chloridoideae; main chloridoid assemblage. Soreng et al. (2015): Chloridoideae; Cynodonteae; Eleusininae. 3 species.
Distribution, phytogeography, ecology. Abyssinia, southern Africa, Indochina, Indomalayan region, Australia.
Mesophytic; shade species and species of open habitats; glycophytic. Sandy savanna.
References, etc. Leaf anatomical: studied by us - A. verticillata Wight & Arn., A. racemosa Ohwi.
Illustrations. • Acrachne racemosa, general aspect and spikelet: Gardner (1952). • A. racemosa, general aspect: Gibbs Russell et al., 1990. • A. verticillata, sculptured grain: Clifford and Watson (1977). • Acrachne verticillata, abaxial epidermis of leaf blade: this project. R.S.B.S. Taxonomy Laboratory (Canberra) slide. Slides and full details now housed at the Western Australian Herbarium.
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: 25th January 2024. delta-intkey.com’.
