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The grass genera of the world

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

Plagiosetum Benth.

From the Greek plagios (oblique) and seta (a bristle), alluding to somewhat unilateral bristles.

Excluding Parectenium

Habit, vegetative morphology. Annual; caespitose to decumbent. Culms 20–50 cm high; herbaceous; branched above. Culm nodes glabrous. Culm internodes hollow. Leaves non-auriculate. Leaf blades narrow; not setaceous; flat, or rolled (convolute); without cross venation; persistent; rolled in bud. Ligule a fringed membrane to a fringe of hairs; 1 mm long.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets all alike in sexuality.

Inflorescence. Inflorescence a false spike, with spikelets on contracted axes (racemose, the cuneate, flattened branches each reduced to 1–2 spikelets and several variously united bristles, reflexing at maturity). Inflorescence axes not ending in spikelets (the racemelet ending in a fan of bristles). Rachides hollowed to flattened. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes very much reduced (with 1–2(-3) spikelets, plus bristles); disarticulating; falling entire (the short branches articulated at the base, falling whole). Spikelets with ‘involucres’ of ‘bristles’ (or at least with the single bristle branched, cf. Parectenium). The ‘bristles’ relatively slender, not spiny; deciduous with the spikelets. Spikelets solitary; secund (the racemes to one side of the main axis).

Female-fertile spikelets. Spikelets 6–7.5 mm long; compressed dorsiventrally; falling with the glumes (the branches falling whole). Rachilla terminated by a female-fertile floret.

Glumes present; two; very unequal; (the longer) long relative to the adjacent lemmas (equalling the spikelet); awnless. Lower glume 3–5 nerved. Upper glume 9–17 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. Spikelets with proximal incomplete florets. The proximal incomplete florets 1. The proximal lemmas awnless; 18–20 nerved; less firm than the female-fertile lemmas (similar to G2); not becoming indurated.

Female-fertile florets 1. Lemmas decidedly firmer than the glumes; rugose (shining); becoming indurated; entire; pointed, or blunt; awnless; hairless; non-carinate; having the margins inrolled against the palea; with a clear germination flap; 3–7 nerved. Palea present; relatively long; entire (apiculate); awnless, without apical setae; textured like the lemma; indurated. Lodicules present; 2; fleshy. Stamens 3. Anthers not penicillate. Ovary apically glabrous. Styles free to their bases. Stigmas 2.

Fruit, embryo and seedling. Fruit small; compressed dorsiventrally. Hilum short. Embryo large. Endosperm containing compound starch grains. Embryo without an epiblast; with a scutellar tail; with an elongated mesocotyl internode. Embryonic leaf margins overlapping.

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 (fairly thin walled). Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; panicoid-type; 45–51–57 microns long; (6.3–)6.9–8.4(–9) microns wide at the septum. Microhair total length/width at septum 5.6–7.4. Microhair apical cells (27–)30–35(–40.5) microns long. Microhair apical cell/total length ratio 0.59–0.71. Stomata common; (24–)27–28.5(–30) microns long. Subsidiaries parallel-sided and dome-shaped. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; not paired (mainly solitary); not silicified. Costal short-cells conspicuously in long rows. Costal silica bodies ‘panicoid-type’; not sharp-pointed.

Transverse section of leaf blade, physiology. C4; XyMS–. PCR sheath outlines uneven. PCR sheath extensions absent. PCR cell chloroplasts centrifugal/peripheral. Mesophyll with radiate chlorenchyma, or with non-radiate chlorenchyma (rarely). Leaf blade adaxially flat. Midrib conspicuous; with one bundle only; with colourless mesophyll adaxially. Bulliforms not present in discrete, regular adaxial groups (the epidermis irregularly bulliform). Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders present; nowhere forming ‘figures’. Sclerenchyma all associated with vascular bundles.

Classification. Watson & Dallwitz (1994): Panicoideae; Panicodae; Paniceae. Soreng et al. (2015): Panicoideae; Panicodae; Paniceae; Cenchrinae. 1 species (P. refractum).

Distribution, phytogeography, ecology. Australia.

Xerophytic; species of open habitats. Dry sandhills.

References, etc. Leaf anatomical: studied by us.

Illustrations. • P. refractum: Gardner, 1952

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.’.