| # The grass genera of the world |

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

### Piresia Swallen

Named for J.M. Pires, Brazilian botanist.

~ Reitzea

Habit, vegetative morphology. Perennial. The flowering culms mostly leafless (and decumbent in the leaf litter, but erect foliage culms also sometimes having a terminal inflorescence). Culms 8–40 cm high; herbaceous; unbranched above. Plants unarmed. Leaves not basally aggregated; without auricular setae. Leaf blades pseudopetiolate; cross veined. Contra-ligule absent.

Reproductive organization. Plants monoecious with all the fertile spikelets unisexual; without hermaphrodite florets. The spikelets of sexually distinct forms on the same plant; female-only and male-only. The male and female-fertile spikelets mixed in the inflorescence (one or two males beneath each female). The spikelets overtly heteromorphic.

Inflorescence. Inflorescence paniculate (depauperate, racemelike); spatheate; a complex of ‘partial inflorescences’ and intervening foliar organs (with spatheoles?). Spikelet-bearing axes ‘racemes’, or paniculate; persistent. Spikelets in triplets, or paired; not secund; not in distinct ‘long-and-short’ combinations.

Female-sterile spikelets. Male spikelets usually lacking glumes, with 3 free, non-penicillate stamens. Rachilla of male spikelets terminated by a male floret. The male spikelets usually without glumes; without proximal incomplete florets. Male florets 1; 3 staminate. The staminal filaments free.

Female-fertile spikelets. Spikelets 5–7 mm long; narrowly elliptic; with conventional internode spacings. Rachilla terminated by a female-fertile floret.

Glumes two; more or less equal; long relative to the adjacent lemmas; pointed; awnless (apiculate); non-carinate; similar (herbaceous). Lower glume 3 nerved, or 5 nerved. Upper glume 3 nerved, or 5 nerved. Spikelets with female-fertile florets only.

Female-fertile florets 1. Lemmas decidedly firmer than the glumes (leathery); becoming indurated; awnless; hairy (appressed pilose); non-carinate; having the margins inrolled against the palea. Palea present; relatively long; entire; awnless, without apical setae; textured like the lemma; indurated; 2-nerved. Stamens 0 (staminodes 0 or three). Styles fused. Stigmas 2.

Fruit, embryo and seedling. Fruit compressed dorsiventrally. Hilum long-linear. Embryo small.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present (and very abundant); costal and intercostal (but far more conspicuous intercostally). Intercostal papillae over-arching the stomata; several per cell (one or two irregular rows of irregularly shaped, rather angular papillae per long-cell). Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; elongated; clearly two-celled; panicoid-type; 24–39 microns long; 3.6–4.5 microns wide at the septum. Microhair total length/width at septum 7.7–10.8. Microhair apical cells 15–17.5 microns long. Microhair apical cell/total length ratio 0.4–0.45. Stomata common; 18–21 microns long. Subsidiaries papillate (commonly two on each); high dome-shaped (mostly), or triangular. Intercostal short-cells common; in cork/silica-cell pairs; silicified. Intercostal silica bodies vertically elongated-nodular. Large, cushion-based macrohairs common. Crown cells absent. Costal short-cells conspicuously in long rows. Costal silica bodies ‘panicoid-type’; mostly Maltese- cross shaped (sometimes almost ‘oryziod’), or butterfly shaped to dumb-bell shaped (a few, short); not sharp-pointed.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with adaxial palisade; with arm cells (the cells adjoining the fusoids with a few large and conspicuous ingrowths); with fusoids. The fusoids external to the PBS. Leaf blade adaxially flat. Midrib conspicuous (by virtue of its large bundle and adaxially prominent rib); with one bundle only. The lamina symmetrical on either side of the midrib. Bulliforms present in discrete, regular adaxial groups; in simple fans (the groups large and wide, one in each intercostal zone). All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; forming ‘figures’ (all the bundles with a small ‘anchor’, except the midrib with strands only). Sclerenchyma all associated with vascular bundles.

Cytology. Chromosome base number, x = 11. 2n = 22. 2 ploid.

Classification. Watson & Dallwitz (1994): Bambusoideae; Oryzodae; Olyreae. Soreng et al. (2015): Bambusoideae; Bambusodae; Olyreae; Olyrinae. 4 species.

Distribution, phytogeography, ecology. Tropical America.

Mesophytic; shade species.

References, etc. Morphological/taxonomic: Swallen 1964; Soderstrom 1982b. Leaf anatomical: studied by us - P. goeldii Swallen.

Illustrations. • Piresia goeldii, abaxial epidermis of leaf blade: this project. • Piresia goeldii, abaxial epidermis of 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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