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The grass genera of the world

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

Dimeria R.Br.

From the Greek dis (double) and meros (part), alluding to species with two racemes.

Including Didactylon Zoll. & Mor., Haplachne Presl, Psilostachys Steud., Pterostachyum Steud., Woodrowia Stapf

Habit, vegetative morphology. Annual, or perennial (often pigmented when mature); caespitose. Culms herbaceous; branched above, or unbranched above. The branching simple. Culm nodes hairy (usually?). Culm internodes hollow. Leaves auriculate, or non-auriculate. Leaf blades linear (acuminate); narrow; without cross venation; persistent. Ligule an unfringed membrane.

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

Inflorescence. Inflorescence of spicate main branches, or a single raceme (the raceme or racemes spiciform); deciduous in its entirety (in D. woodrowii, the two mature racemes coil into a ball and are shed together), or not deciduous; when of more than one raceme, digitate. Primary inflorescence branches 1–9. Rachides hollowed (triquetrous), or flattened. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes ‘racemes’; with very slender rachides, or with substantial rachides (filiform or flattened); persistent (the rachis tough). Spikelets solitary; secund; biseriate; shortly pedicellate (the pedicels usually but not always shortly clavate); imbricate, or not imbricate; emphatically not in distinct ‘long-and-short’ combinations (despite being obviously ‘andropogonoid’).

Female-fertile spikelets. Spikelets strongly compressed laterally; falling with the glumes, or not disarticulating (D. woodrowii). Hairy callus present (shortly bearded).

Glumes two; more or less equal; awnless; carinate; with the keel conspicuously winged, or without a median keel-wing; similar. Lower glume not two-keeled; not pitted; relatively smooth; 1–3 nerved. Upper glume 1–3 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. Spikelets with proximal incomplete florets. The proximal incomplete florets 1; epaleate; sterile. The proximal lemmas awnless; 0 nerved; similar in texture to the female-fertile lemmas (thinly membranous to hyaline).

Female-fertile florets 1. Lemmas less firm than the glumes; not becoming indurated; incised (or rarely entire); awnless, or awned. Awns when present, 1; usually from a sinus; geniculate; hairless (glabrous); much longer than the body of the lemma. Lemmas hairless; without a germination flap; 1–3 nerved. Palea absent. Lodicules absent. Stamens usually 2 (the filaments short). Anthers not penicillate. Ovary apically glabrous. Styles fused. Stigmas 2.

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

Seedling with a long mesocotyl.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present; costal and intercostal. Intercostal papillae not over-arching the stomata; several per cell (one longitudinal row per cell). Long-cells similar in shape costally and intercostally (narrowly so); of similar wall thickness costally and intercostally (thin walled). Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; more or less spherical, or elongated; ostensibly one-celled, or clearly two-celled; panicoid-type and chloridoid-type; 9–18 microns long; (7.2–)7.5–8.4(–9) microns wide at the septum. Microhair total length/width at septum 1–3. Microhair apical cells 7.5–9(–10.5) microns long. Microhair apical cell/total length ratio 0.5–0.6. Stomata common; 18–19.5 microns long. Subsidiaries papillate; triangular. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells absent or very rare; not paired (very rare); when seen not silicified. Costal short-cells conspicuously in long rows. Costal silica bodies horizontally-elongated crenate/sinuous (almost), or ‘panicoid-type’ (mostly); cross shaped, or dumb-bell shaped, or nodular.

Transverse section of leaf blade, physiology. C4; XyMS–. Mesophyll with radiate chlorenchyma. Leaf blade adaxially flat. Midrib conspicuous; having a conventional arc of bundles; with colourless mesophyll adaxially. Bulliforms not present in discrete, regular adaxial groups (irregularly grouped in the largely bulliform epidermis). Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders present; forming ‘figures’. Sclerenchyma all associated with vascular bundles.

Culm anatomy. Culm internode bundles in one or two rings, or in three or more rings.

Phytochemistry. Leaves without flavonoid sulphates (1 species).

Cytology. 2n = 14. Chromosomes ‘small’.

Classification. Watson & Dallwitz (1994): Panicoideae; Andropogonodae; Andropogoneae; Andropogoninae. Soreng et al. (2015): Panicoideae; Andropogonodae; Andropogoneae; Dimeriinae. 40 species.

Distribution, phytogeography, ecology. Mascarene Is., Southeast Asia, Indomalayan region, Australia, Polynesia.

Mesophytic; species of open habitats. Forest glades and margins.

Rusts and smuts. Rusts — Phakopsora. Taxonomically wide-ranging species: Phakopsora incompleta. Smuts from Ustilaginaceae. Ustilaginaceae — Sorosporium.

References, etc. Leaf anatomical: Metcalfe 1960; studied by us - ‘Dimeria sp.’.

Illustrations. • D. ornithopoda: 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.’.