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The grass genera of the world

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

Zygochloa S. T. Blake

From the Greek zygon (yoke, or pair) and chloa (grass), referring to dioecious spikelets.

Type species: Type: Z. paradoxa (R.Br.) S.T.Blake.

Habit, vegetative morphology. Shrubby perennial (‘cane-grass’); rhizomatous (and tussock-forming). Culms 80–200 cm high; woody and persistent; to 0.9 cm in diameter (near the base); grooved on one side; branched above. The branching suffrutescent. Culm nodes glabrous. Culm leaf sheaths rounded. Culm internodes solid. Young shoots intravaginal. Leaves not basally aggregated; non-auriculate. Leaf blades broad to narrow; 4–10 mm wide (to 30 cm long); flat (short, stiff); without cross venation; disarticulating from the sheaths. Ligule a fringe of hairs.

Reproductive organization. Plants dioecious; without hermaphrodite florets. The spikelets all alike in sexuality (on the same plant); female-only, or male-only. Plants outbreeding.

Inflorescence. Inflorescence a spatheate panicle of ‘bracteate’ heads; open. Inflorescence axes not ending in spikelets (interpreting one of the ‘bracts’ as a modified axis tip). Inflorescence spatheate (and ‘bracteate’); a complex of ‘partial inflorescences’ and intervening foliar organs. Spikelet-bearing axes very much reduced (to a single spikelet accompanied by two basal bracts and the small, naked, bractlike axis tip, the units grouped into capitate heads); disarticulating; falling entire (the heads falling). Spikelets associated with bractiform involucres (each spikelet associated with the three chaffy, rigid-tipped ‘bracts’). The involucres shed with the fertile spikelets. Spikelets solitary; not secund; subsessile. Pedicel apices discoid. Spikelets not in distinct ‘long-and-short’ combinations.

Female-sterile spikelets. Male spikelets on separate individuals, in small bracteate heads 1–2 cm in diameter. Glumes similar, 5–7 nerved; lemmas 2, similar, awnless, 5 nerved, each with a male floret. Paleas conspicuous, with winged keels. 3 stamens, anthers 4 mm long, no gynoecium. Rachilla of male spikelets terminated by a male floret. The male spikelets with glumes; without proximal incomplete florets; 2 floreted (both fertile). The lemmas awnless. Male florets 2; 3 staminate.

Female-fertile spikelets. Spikelets 6–10 mm long; lanceolate, or ovate; compressed dorsiventrally; falling with the glumes. Rachilla terminated by a female-fertile floret. Hairy callus absent.

Glumes two; more or less equal; shorter than the adjacent lemmas; pointed, or not pointed; awnless; similar (papery). Lower glume 7–9 nerved. Upper glume 5–7 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. Spikelets with proximal incomplete florets. The proximal incomplete florets 1; sterile. The proximal lemmas awnless; 5 nerved; becoming indurated.

Female-fertile florets 1. Lemmas abruptly acuminate; decidedly firmer than the glumes; smooth, or striate; becoming indurated (crustaceous); yellow in fruit; entire; pointed; awnless; hairless; non-carinate; having the margins inrolled against the palea; 5 nerved. Palea present; entire (ovate, abruptly acuminate); awnless, without apical setae; textured like the lemma; 2-nerved. Lodicules present; 2; fleshy. Stamens 0 (3 staminodes). Ovary apically glabrous. Styles fused. Stigmas 2.

Fruit, embryo and seedling. Fruit small (about 3 mm long). Hilum short.

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, or absent (not seen on male material); chloridoid-type; about 66 microns long; 9 microns wide at the septum. Microhair total length/width at septum 7.3. Microhair apical cells about 42 microns long. Microhair apical cell/total length ratio 0.64. Stomata common; 42–57 microns long. Subsidiaries parallel-sided, dome-shaped, and triangular; including both triangular and parallel-sided forms on the same leaf. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; in cork/silica-cell pairs and not paired (solitary); silicified (when paired), or not silicified. Costal zones with short-cells. 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. Mesophyll with radiate chlorenchyma. Leaf blade ‘nodular’ in section; with the ribs more or less constant in size. Midrib not readily distinguishable; with one bundle only. Bulliforms present in discrete, regular adaxial groups (the epidermis extensively, irregularly bulliform). Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders present; forming ‘figures’. Sclerenchyma all associated with vascular bundles.

Special diagnostic feature. Stems cane-like, spikelets in bracteate, globular 1–3.5 cm heads.

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

Distribution, phytogeography, ecology. Australia.

Xerophytic; species of open habitats. Arid sandy and rocky places.

References, etc. Morphological/taxonomic: Vickery 1975; Webster 1987. Leaf anatomical: studied by us.

Special comments. Fruit data wanting. Illustrations. • Z. paradoxa, as Spinifex: Hook. Ic. Pl. 13 (1877–79). • Z. paradoxa: Lazarides (1981). • Abaxial epidermis of leaf blade of Z. paradoxa, male plant: this project. • Abaxial epidermis of leaf blade of Z. paradoxa, female plant: this project

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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.’.