Coelorachis Brongniart.

Perennial, rarely annual. Culms robust, tufted, often branched. Leaf blades linear, flat; ligule short, membranous. Inflorescence of racemes, these usually axillary and aggregated into a spathate compound panicle, rarely terminal. Racemes cylindrical or flattened, fragile, horizontally articulated; rachis internodes clavate to pyriform, base truncate with central peg, spikelets paired, one sessile, the other pedicelled or occasionally in triplets of 2 sessile and 1 pedicelled. Sessile spikelet closely appressed or sunk in hollow in rachis, usually longer than adjacent internode; lower glume papery to leathery, ± flat, smooth or sculptured, marginally 2-keeled, keels winged at least at apex; lower floret barren, with or without a small palea; upper floret with entire awnless lemma. Pedicelled spikelet varying from well developed to rudimentary or absent; pedicel oblong, clavate or leaflike, free or partially or fully adnate to rachis internode. x = 9.

Mnesithea has been defined in the past by the presence of 2 sessile spikelets separated by a pedicel at each rachis node, but this character is now known to be variable. Furthermore, paired sessile spikelets also occur sporadically in the racemes of species that normally have single sessile spikelets.

Coelorachis has traditionally been separated on the basis of a free pedicel and presence of a pedicelled spikelet, but both these characters are untenable. The pedicel in Mnesithea khasiana is often almost completely fused to the rachis joint and may occasionally be fused in other species too. The pedicelled spikelet varies across the genus from well developed to a minute vestige, and its absence in traditional Mnesithea is just the final step.

The definition of the genera around Mnesithea, and how many species should be included within it, are still open to doubt and may change with future research.

About 30 species: throughout the tropics; four species in China.

(Authors: Sun Bixing (孙必兴 Sun Bi-sin); Sylvia M. Phillips)