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The grass genera of the world

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

Potamophila R.Br.

From the Greek potamos (river) and -philus (loving), alluding to the habitat of the only species.

Type species: Type: P. parviflora R.Br.

Habit, vegetative morphology. Perennial; rhizomatous and caespitose. The flowering culms leafy. Culms 30–150 cm high; herbaceous. Culm nodes glabrous. Culm internodes hollow. Young shoots extravaginal. Leaves not basally aggregated; auriculate, or non-auriculate; without auricular setae. Leaf blades linear; narrow; 4–6 mm wide; not pseudopetiolate; without cross venation; disarticulating from the sheaths, or persistent; rolled in bud. Ligule an unfringed membrane; not truncate (acute or lacerate); 4–15 mm long. Contra-ligule absent.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets of sexually distinct forms on the same plant (hermaphrodite and unisexual); hermaphrodite and female-only, or hermaphrodite and male-only; homomorphic.

Inflorescence. Inflorescence paniculate; open (to loosely contracted); with capillary branchlets; espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelets not secund; pedicellate.

Female-fertile spikelets. Spikelets unconventional (by virtue of reduced glumes, cf. Oryza); 3–5.5 mm long; oblong, or lanceolate; strongly compressed laterally; disarticulating above the glumes (i.e., above the tiny cupule representing them); with conventional internode spacings (i.e. the floret not stipitate). Rachilla terminated by a female-fertile floret. Hairy callus absent.

Glumes present to absent (vestigial); if considered present, two; minute (reduced to a cupular or bilobed rim); more or less equal; shorter than the adjacent lemmas; joined; awnless. Lower glume 0 nerved. Upper glume 0 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. Spikelets with proximal incomplete florets. The proximal incomplete florets 2; epaleate; sterile. The proximal lemmas ovate; awnless; 0 nerved; exceeded by the female-fertile lemmas (very small, less than 1/8 to 1/4 the spikelet length); less firm than the female-fertile lemmas to similar in texture to the female-fertile lemmas (membranous); not becoming indurated.

Female-fertile florets 1. Lemmas membranous or thinly chartaceous; not becoming indurated; entire; pointed to blunt; awnless; hairless (often scaberulous on the sides); non-carinate (‘sub-keeled’); 5 nerved. Palea present; relatively long; not clasped by the lemma; entire; awnless, without apical setae; textured like the lemma; not indurated; several nerved (three nerved); one-keeled. Lodicules present; 2; free; membranous; glabrous. Stamens 6. Ovary apically glabrous. Styles free to their bases. Stigmas 2.

Fruit, embryo and seedling. Fruit compressed dorsiventrally to not noticeably compressed. Hilum long-linear. Embryo small; with an epiblast; without a scutellar tail; with a negligible mesocotyl internode. Embryonic leaf margins meeting.

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; panicoid-type; (34.5–)39–45(–51) microns long; 7.5–8.4–9 microns wide at the septum. Microhair total length/width at septum 4.3–6.1. Microhair apical cells (12–)16.5–21(–22.5) microns long. Microhair apical cell/total length ratio 0.35–0.47. Stomata common; 19.5–21 microns long. Subsidiaries dome-shaped (mostly), or dome-shaped. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; in cork/silica-cell pairs, or not paired; silicified, or not silicified. Costal short-cells predominantly paired, or neither distinctly grouped into long rows nor predominantly paired (rarely). Costal silica bodies oryzoid to ‘panicoid-type’ (mostly), or tall-and-narrow (a few); not sharp-pointed.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with non-radiate chlorenchyma; without adaxial palisade; with arm cells; with fusoids. The fusoids external to the PBS (short). Leaf blade with distinct, prominent adaxial ribs; with the ribs more or less constant in size, or with the ribs very irregular in sizes. Midrib conspicuous; having complex vascularization. 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.

Classification. Watson & Dallwitz (1994): Bambusoideae; Oryzodae; Oryzeae. Soreng et al. (2015): Oryzoideae; Oryzeae; Zizaniinae. 1 species (P. parviflora).

Distribution, phytogeography, ecology. Australia.

Helophytic; glycophytic.

References, etc. Leaf anatomical: studied by us.

Illustrations. • P. parviflora: Trinius, Spec. Graminum 3 (1830). • P. parviflora: Kunth (1835), Dist. méthodique de la famille Graminées. • P. parviflora, TS leaf blade: this project

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.’.