The grass genera of the world
~ Potamophila (P. prehensilis)
Habit, vegetative morphology. Perennial; rhizomatous (the rhizomes horizontal, with cataphylls). The flowering culms leafy. Culms 1000 cm high (or more); herbaceous; scandent (by retrorse hairs on the leaf blade margins); branched above. Culm nodes hairy, or glabrous. Culm internodes hollow. Plants unarmed (but leaf margins and main veins spiny, with minute retrorse barbs). Young shoots intravaginal. Leaves not basally aggregated; auriculate (sheaths slightly auricled); without auricular setae. Leaf blades linear-lanceolate to lanceolate; narrow to broad; 4–15 mm wide; flat; without cross venation; persistent; rolled in bud. Ligule an unfringed membrane; not truncate (rounded, the margin lacerate-fimbriate); 1–1.5 mm long. Contra-ligule absent.
Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets.
Inflorescence. Inflorescence paniculate (terminating main culm and laterals, the branchlets fine and stiff); open; espatheate; not comprising partial inflorescences and foliar organs. Spikelet-bearing axes persistent. Spikelets solitary; not secund; pedicellate.
Female-fertile spikelets. Spikelets unconventional (by virtue of the reduced glumes); 6–9 mm long; elliptic; slightly compressed laterally; disarticulating above the glumes (i.e. above the rudimentary glumes); not disarticulating between the florets. Rachilla terminated by a female-fertile floret. Hairy callus absent. Callus absent.
Glumes present, or absent; two (reduced to a bilobed to entire hyaline cup); minute; more or less equal; shorter than the spikelets; shorter than the adjacent lemmas; joined; hairless; glabrous; not pointed (blunt); awnless. Lower glume 0 nerved. Upper glume 0 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. The proximal incomplete florets 2; epaleate; sterile (subulate, edged with minute hyaline spines, variable in size). The proximal lemmas awnless; 1 nerved; exceeded by the female-fertile lemmas (very short).
Female-fertile florets 1. Lemmas acuminate; chartaceous or leathery; entire; pointed; awnless; hairless; scabrous; carinate (the nerves with prickles); without a germination flap; 5 nerved; with the nerves confluent towards the tip. Palea present (similar to the lemma, which clasps it); relatively long; tightly clasped by the lemma; entire (acuminate); awnless, without apical setae; textured like the lemma; leathery; several nerved (3); one-keeled. Lodicules present; 2; free; membranous (above, but fleshy below); glabrous; not toothed; heavily vascularized. Stamens 6. Anthers 5 mm long; not penicillate; without an apically prolonged connective. Ovary apically glabrous. Styles free to their bases. Stigmas 2 (plumose); white.
Fruit, embryo and seedling. Fruit free from both lemma and palea; medium sized (5 to 6 mm long, brown); fusiform; longitudinally grooved; not noticeably compressed; longitudinally ribbed. Hilum long-linear. Embryo small. Endosperm containing compound starch grains. Embryo with an epiblast; without a scutellar tail.
Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present. Intercostal papillae several per cell (one or two rows of small, circular papillae on each long-cell, and the guard-cells overtopped by four small papillae, two from each subsidiary). Long-cells markedly different in shape costally and intercostally (the costals relatively long and narrow); of similar wall thickness costally and intercostally (walls of medium thickness). Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; panicoid-type; 45–48 microns long; 5.4–6 microns wide at the septum. Microhair total length/width at septum 7.5–8.9. Microhair apical cells (19.5–)21–22(–22.5) microns long. Microhair apical cell/total length ratio 0.41–0.49. Stomata common; 21–22.5 microns long. Subsidiaries papillate; dome-shaped to triangular. Guard-cells overlapped by the interstomatals (slightly). Intercostal short-cells absent or very rare. Costal short-cells conspicuously in long rows. Costal silica bodies oryzoid; not sharp-pointed.
Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with non-radiate chlorenchyma; without adaxial palisade; with arm cells; with fusoids (i.e. with lateral sheath extensions), or without fusoids (if these not so interpreted). The fusoids an integral part of the PBS. Leaf blade adaxially flat. Midrib conspicuous; having complex vascularization (there being a small bundle adaxial to the main one); with colourless mesophyll adaxially. The lamina symmetrical on either side of the midrib. Bulliforms present in discrete, regular adaxial groups; in simple fans (the fans broad). All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; forming figures. Sclerenchyma all associated with vascular bundles.
Special diagnostic feature. Scandent via leaf blades with retrorsely scabrid margins.
Cytology. Chromosome base number, x = 12. 2n = 24. 2 ploid.
Classification. Watson & Dallwitz (1994): Bambusoideae; Oryzodae; Oryzeae. Soreng et al. (2015): Oryzoideae; Oryzeae; Zizaniinae. 1 species (P. prehensilis).
Distribution, phytogeography, ecology. South Africa.
Helophytic; shade species; glycophytic.
References, etc. Morphological/taxonomic: de Winter 1951. Leaf anatomical: studied by us.
Illustrations. • General aspect (P. prehensilis): Gibbs Russell et al., 1990
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. delta-intkey.com/grass’.