| # The grass genera of the world |

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

### Mesosetum Steud.

Including Bifaria (Hack.) Kuntze, Peniculus Swallen

Habit, vegetative morphology. Annual, or perennial; stoloniferous, or caespitose. Culms herbaceous; branched above, or unbranched above. The branching simple. Culm nodes hairy. Culm internodes hollow. Leaves mostly basal, or not basally aggregated; non-auriculate. Leaf blades narrow; flat, or rolled (convolute); without cross venation; persistent. Ligule a fringed membrane, or a fringe of hairs.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets.

Inflorescence. Inflorescence a single spike, or a single raceme (with subsessile spikelets). Rachides hollowed to flattened, or winged (membranously, in M. ansatum). Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets solitary; not secund; biseriate; subsessile.

Female-fertile spikelets. Spikelets adaxial; compressed dorsiventrally; falling with the glumes. Rachilla terminated by a female-fertile floret. Hairy callus present, or absent.

Glumes two; very unequal; (the upper) long relative to the adjacent lemmas; dorsiventral to the rachis; hairy (the upper only, or both), or hairless; (the lower) awned (or mucronate, sometimes), or awnless; carinate (the upper strongly keeled, slightly winged). Lower glume 3 nerved. Upper glume (3–)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; paleate, or epaleate. Palea of the proximal incomplete florets when present, reduced (hyaline). The proximal incomplete florets sterile. The proximal lemmas usually hyaline down the middle; awnless; 5 nerved; decidedly exceeding the female-fertile lemmas; less firm than the female-fertile lemmas; not becoming indurated.

Female-fertile florets 1. Lemmas similar in texture to the glumes to decidedly firmer than the glumes (leathery); smooth; not becoming indurated; entire; acute to mucronate; awnless; hairless; glabrous; non-carinate; having the margins lying flat on the palea; with a clear germination flap; 5 nerved. Palea present; relatively long; entire; awnless, without apical setae; textured like the lemma (apart from the hyaline margins); 2-nerved; keel-less. Lodicules present; 2; free; fleshy; glabrous; not or scarcely vascularized. Stamens 3. Anthers not penicillate. Ovary apically glabrous. Styles free to their bases. Stigmas 2; red pigmented.

Fruit, embryo and seedling. Fruit free from both lemma and palea; small; longitudinally grooved; compressed laterally (slightly), or not noticeably compressed. Hilum long-linear (at least in M. pittieri). Embryo large to small; not waisted. Endosperm hard.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Mid-intercostal long-cells having markedly sinuous walls. Microhairs present; ostensibly one-celled (the apical cells bulbous-based with pointed tips, seemingly lying horizontally over short, embedded basal cells); small, peculiar and hard to interpret in both species seen - cf. Coelachne, Coelachyrum?. Stomata common; 43.5–54 microns long. Subsidiaries triangular. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; in cork/silica-cell pairs; silicified. Costal short-cells conspicuously in long rows (M. loliiforme), or neither distinctly grouped into long rows nor predominantly paired (M. pittieri). Costal silica bodies ‘panicoid-type’; mostly cross shaped; not sharp-pointed.

Transverse section of leaf blade, physiology. C4; XyMS– (but variable in bundles of M. loliiforme). Mesophyll with radiate chlorenchyma. Leaf blade ‘nodular’ in section; with the ribs more or less constant in size. Midrib not readily distinguishable (slightly larger); having a conventional arc of bundles (1 large, 2 small). Bulliforms present in discrete, regular adaxial groups; in simple fans. Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders present (M. loliforme), or absent (M. pittieri); nowhere forming ‘figures’. Sclerenchyma all associated with vascular bundles (excluding the blade margins).

Cytology. 2n = 16.

Classification. Watson & Dallwitz (1994): Panicoideae; Panicodae; Paniceae. Soreng et al. (2015): Panicoideae; Panicodae; Paspaleae; Arthropogoninae. 35 species.

Distribution, phytogeography, ecology. Central & tropical South America, West Indies.

Neotropical.

Species of open habitats. Savanna.

Rusts and smuts. Smuts from Ustilaginaceae. Ustilaginaceae — Sphacelotheca.

References, etc. Leaf anatomical: studied by us - M. altum Swallen, M. loliiforme (Hochst. ex Steud.) Hitchc., M. pittieri Hitchc.

Illustrations. • Mesosetum loliiforme: Hitchcock (1936), W.I. Grasses.. • Mesosetum comatum: Nicora & Rúgolo de Agrasar (1987).

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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