| # The grass genera of the world |

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

### Thuarea Pers.

After the French botanist du Petit Thouars.

Type species: Type: T. involuta (G.Forst.) Roem. & Schult.

Including Ornithocephalochloa Kurz, Microthuareia Thouars, Thouarsia Kuntze

Habit, vegetative morphology. Perennial; decumbent (creeping, mat-forming). Culms 5–30 cm high; herbaceous; unbranched above. Culm nodes glabrous. Culm leaf sheaths rounded. Culm internodes solid. Plants unarmed. Young shoots intravaginal. Leaves not basally aggregated; non-auriculate. Leaf blades linear-lanceolate (broad-based); broad, or narrow; without cross venation; persistent; rolled in bud. Ligule a fringe of hairs.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets, or without hermaphrodite florets. The spikelets of sexually distinct forms on the same plant; hermaphrodite and male-only, or hermaphrodite, female-only, and male-only, or female-only and male-only (the lower 1–2 spikelets hermaphrodite or female, the upper 2–6 male-only). The male and female-fertile spikelets segregated, in different parts of the same inflorescence branch (the male spikelets distal to the female-fertile ones on the rachis). The spikelets homomorphic.

Inflorescence. Inflorescence a peculiar spiciform ‘raceme’, terminal, shortly peduncled, broad and winglike in the female-fertile part, narrow and beak-like in the upper male part. Rachides flattened (below). Inflorescence spatheate (the young inflorescence enclosed in the spathaceous blade of the uppermost culm leaf); not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes spikelike; solitary; persistent. Spikelets discernably paired; secund; not two-ranked (in one row, on the channelled side of the rachis); discernably sessile and pedicellate (though the pedicels fused); consistently in ‘long-and-short’ combinations. Pedicels of the ‘pedicellate’ spikelets discernible, but fused with the rachis.

Female-sterile spikelets. Male spikelets in the upper part of the raceme, articulated with their bulbous pedicels, deciduous. Both or only the upper of the two florets male, with 3 stamens. Rachilla of male spikelets terminated by a male floret. The male spikelets with proximal incomplete florets, or without proximal incomplete florets; 2 floreted (both or only one fertile). Male florets 1, or 2; 3 staminate.

Female-fertile spikelets. Spikelets elliptic, or lanceolate, or ovate; adaxial; compressed dorsiventrally; not disarticulating (after anthesis the upper (male) spikelets fall, then the axis bends to enclose the developing fruit, and the flowering branch bends down to thrust the ripened seed into the sand). Rachilla terminated by a female-fertile floret. Hairy callus absent.

Glumes one per spikelet to two (the G1 when present minute, hyaline); shorter than the adjacent lemmas; dorsiventral to the rachis; the upper softly hairy; not pointed (obtuse); awnless; non-carinate; very dissimilar (when both present, the lower vestigial). Lower glume when present, 0 nerved. Upper glume 5 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. The proximal incomplete florets 1; paleate. Palea of the proximal incomplete florets fully developed. The proximal incomplete florets male. The proximal lemmas awnless; 5–7 nerved; more or less equalling the female-fertile lemmas; less firm than the female-fertile lemmas; not becoming indurated.

Female-fertile florets 1. Lemmas decidedly firmer than the glumes; smooth; papery; yellow in fruit; entire; blunt; awnless; hairy (apically, otherwise glabrous); non-carinate; having the margins lying flat on the palea; with a clear germination flap; 5–9 nerved. Palea present; relatively long; entire; awnless, without apical setae; textured like the lemma; 2-nerved; 2-keeled. Lodicules present; 2; free; fleshy; glabrous. Stamens 3. Anthers not penicillate; without an apically prolonged connective. Ovary apically glabrous. Styles fused, or free to their bases. Stigmas 2; white.

Fruit, embryo and seedling. Fruit small to medium sized (3–4 mm long); compressed dorsiventrally. Hilum short. Embryo large; without an epiblast; with a scutellar tail; with an elongated mesocotyl internode; with one scutellum bundle. Embryonic leaf margins overlapping.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Intercostal zones without typical long-cells (except for occasional typical long-cells mid-way between the more widely spaced veins). Mid-intercostal long-cells having straight or only gently undulating walls. Microhairs present (but sometimes scarce, and hard to find among the macrohairs); elongated; clearly two-celled; panicoid-type; 42–55–67.5 microns long; 5.4–6.2–6.9 microns wide at the septum. Microhair total length/width at septum 7.7–10. Microhair apical cells 30–37–45 microns long. Microhair apical cell/total length ratio 0.59–0.73. Stomata common; 21–27 microns long. Subsidiaries dome-shaped. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells absent or very rare. Macrohairs abundant. Costal short-cells conspicuously in long rows, or neither distinctly grouped into long rows nor predominantly paired. Costal silica bodies poorly developed, or absent (in the material seen); when present, probably ‘panicoid-type’ (judging from the silica cell shapes); presumably dumb-bell shaped; 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; traversed by columns of colourless mesophyll cells (rarely), or not traversed by colourless columns. Leaf blade adaxially flat; with the ribs more or less constant in size. Midrib not readily distinguishable; with one bundle only. Bulliforms present in discrete, regular adaxial groups; in simple fans. Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders absent. Sclerenchyma all associated with vascular bundles.

Phytochemistry. Leaves without flavonoid sulphates (T. involuta).

Special diagnostic feature. Spikelets not borne on a broad, leaflike rachis (the flattened rachis not leaflike). Flowering culms ultimately bending over, so as to enclose the ripening fruit.

Classification. Watson & Dallwitz (1994): Panicoideae; Panicodae; Paniceae. Soreng et al. (2015): Panicoideae; Panicodae; Paniceae; Melinidinae. 2 species.

Distribution, phytogeography, ecology. 1 in Madagascar, 1 Indomalaya, North Australia, New Guinea.

Commonly adventive. Species of open habitats; halophytic. Seashore sand.

Economic aspects. Significant weed species: T. involuta. Important native pasture species: T. involuta.

References, etc. Leaf anatomical: studied by us - T. involuta (Forster f.) R. Br. ex Roem. & Schultes.

Illustrations. • Thuarea involuta, as T. sarmentosa: Kunth (1835). • Thuarea involuta, abaxial epidermis of leaf blade: this project. • Thuarea involuta, abaxial epidermis of leaf blade: this project. • Thuarea involuta, abaxial epidermis of leaf blade: this project. • Thuarea involuta, 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: 15th June 2025. delta-intkey.com’.

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