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The grass genera of the world

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

Isachne R.Br.

From the Greek isos (equal) and achne (chaff), referring to two equal florets, or equal glumes, or glumes equalling lemmas, or all these in combination?.

Type species: Type: I. globosa (Thunb.) O.Kuntz.

Habit, vegetative morphology. Annual (rarely), or perennial (often aquatic); rhizomatous, or stoloniferous, or caespitose, or decumbent. Culms 5–100 cm high; herbaceous (rarely almost shrubby); scandent (to 6 m in I. arundinacea), or not scandent; sparsely to amply branched above, or unbranched above. The branching simple, or fastigiate (rarely). Culm nodes glabrous. Culm leaf sheaths rounded. Culm internodes hollow. Leaves not basally aggregated; non-auriculate. Leaf blades linear-lanceolate to ovate, or linear (rarely); broad, or narrow; 0.5–12(–20) mm wide; cordate (occasionally - I. polygonoides), or not cordate, not sagittate; not pseudopetiolate; without cross venation; disarticulating from the sheaths. Ligule present, or absent; a fringed membrane (very narrow), or a fringe of hairs. Contra-ligule present (of hairs), or absent.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets, or without hermaphrodite florets.

Inflorescence. Inflorescence paniculate; open; with capillary branchlets, or without capillary branchlets. Primary inflorescence branches inserted all around the main axis. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets not secund. Pedicel apices cupuliform.

Female-fertile spikelets. Spikelets 0.6–2 mm long; elliptic, or ovate, or obovate; compressed dorsiventrally; disarticulating above the glumes; not disarticulating between the florets, or disarticulating between the florets. Rachilla terminated by a female-fertile floret; hairless. Hairy callus absent.

Glumes two; more or less equal; long relative to the adjacent lemmas; awnless; similar (membranous). Lower glume 3–9 nerved. Upper glume 5–9 nerved. Spikelets with female-fertile florets only, or with incomplete florets. The incomplete florets (when present) proximal to the female-fertile florets, or distal to the female-fertile florets (the distal floret may be female-only, and rarely the proximal floret is male-only). The proximal incomplete florets when present, 1; when present, paleate. Palea of the proximal incomplete florets fully developed. The proximal incomplete florets when present, male. The proximal lemmas variable - similar to L2 or much longer and membranous; awnless.

Female-fertile florets 1 (occasionally), or 2 (usually - typically with both florets perfect). Lemmas orbicular to broadly elliptic; similar in texture to the glumes to decidedly firmer than the glumes (chartaceous to leathery); smooth to striate; not becoming indurated; entire; pointed, or blunt; awnless; hairy (pubescent), or hairless (glabrous); non-carinate; having the margins inrolled against the palea; with a clear germination flap; 5–7 nerved. Palea present; relatively long; entire; awnless, without apical setae; textured like the lemma; not indurated; 2-nerved. Lodicules present; 2; free; fleshy; glabrous. Stamens 3. Anthers not penicillate. Ovary apically glabrous. Styles free to their bases. Stigmas 2; red pigmented.

Fruit, embryo and seedling. Fruit small; compressed dorsiventrally. Hilum long-linear. Embryo large. Endosperm containing only simple starch grains. Embryo without an epiblast; with a scutellar tail; with an elongated 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 broad; curved; 6–12 veined.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present, or absent. Intercostal papillae not over-arching the stomata; consisting of one symmetrical projection per cell (nearly all the very short ‘long-cells’ and interstomatals with a single papilla occupying most of the outer surface). Long-cells markedly different in shape costally and intercostally (the intercostals more or less hexagonal); of similar wall thickness costally and intercostally. Intercostal zones without typical long-cells (the long-cells more or less cubical or even vertically elongated). Mid-intercostal long-cells having markedly sinuous walls, or having straight or only gently undulating walls. Microhairs present; panicoid-type (though the distal cell is rather thick-walled). Stomata common. Subsidiaries parallel-sided to triangular; including both triangular and parallel-sided forms on the same leaf. Guard-cells overlapped by the interstomatals, or overlapping to flush with the interstomatals. Intercostal short-cells absent or very rare. Costal short-cells conspicuously in long rows. Costal silica bodies acutely-angled (more or less cubical); sharp-pointed.

Transverse section of leaf blade, physiology. C3; XyMS+. PBS cells without a suberised lamella. Mesophyll with radiate chlorenchyma; Isachne-type. Leaf blade ‘nodular’ in section; with the ribs more or less constant in size. Midrib conspicuous; with one bundle only. Bulliforms present in discrete, regular adaxial groups (in the shallow furrows); in simple fans. All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; forming ‘figures’, or nowhere forming ‘figures’. Sclerenchyma all associated with vascular bundles.

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

Phytochemistry. Leaves without flavonoid sulphates (2 species).

Cytology. Chromosome base number, x = 10. 2n = 20, 50, and 60. 2, 5, and 6 ploid.

Classification. Watson & Dallwitz (1994): Panicoideae; Panicodae; Isachneae. Soreng et al. (2015): Micrairoideae; Isachneae. About 100 species.

Distribution, phytogeography, ecology. Tropical and subtropical.

Helophytic, or mesophytic; shade species, or species of open habitats; glycophytic. Mostly in marshy ground.

Economic aspects. Significant weed species: I. globosa (in rice). Important native pasture species: in wet places, e.g. I. albens, I. globosa.

Rusts and smuts. Rusts — Puccinia. Smuts from Ustilaginaceae. Ustilaginaceae — Sphacelotheca and Ustilago.

References, etc. Leaf anatomical: Metcalfe 1960; studied by us - I. confusa Ohwi, I. globosa (Thunb.) Kuntze, I. pulchella Roth. ex R. & S.

Illustrations. • I. mauritiana: Kunth (1835). • I. globosa, as I. minutula: Kunth (1835). • I. globosa, close-up of spikelet showing details: this project. Isachne globosa. Similar, equal glumes enclosing two florets with shiny, indurated lemmas; stamens exserted from the proximal floret, red stigmas from the distal one. • I. globosa, abaxial epidermis of leaf blade: this project. • I. globosa, abaxial epidermis of leaf blade: this project. • I. globosa, abaxial epidermis of leaf blade detailing silica bodies: this project. • I. globosa, leaf blade TS: this project. • I. albens: Trinius (1828), Species Graminum I

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: 11th December 2017.’.