Plants perennial; cespitose or rhizomatous. Culms 60-400 cm, erect. Leaves not aromatic; basal and cauline; sheaths open, glabrous, margins scarious; auricles lacking; ligules membranous, ciliate; blades flat to conduplicate, glabrous or sparsely pubescent, margins scarious, sometimes scabrous. Inflorescences terminal and axillary, composed of a solitary, pedunculate rame; rames stout; disarticulation in the rames, below the sessile spikelets. Spikelets dorsally compressed, in heterogamous sessile-pedicellate pairs. Sessile spikelets embedded in the rame axes, ovate, with 2 florets, unawned; lower glumes indurate, smooth, rugose, or pitted, 7-11-veined, not keeled; upper glumes coriaceous, keeled, 1-veined; lower florets sterile; upper florets bisexual, unawned; anthers 3. Pedicels short, thick, appressed or partly fused to the side of the rame axes. Pedicellate spikelets 1-3 mm, usually reduced. Caryopses ellipsoid to broadly ellipsoid, yellow. x = 9. Name from the Greek koilos, hollow, and rachis, axis, in reference to the axes of the inflorescence, which are concave.

Coelorachis is a tropical genus of approximately 20 species; four are native to the southeastern United States. Most species tend to favor damp soils. Veldkamp et al. (1986) recommended combining Coelorachis and Hackelochloa with some other small genera in Mnesithea Kunth, but these two seem to be sufficiently distinct to be maintained until more data are available.

1.   Coelorachis cylindrica (Michx.) Nash
Carolina Jointgrass

Plants shortly rhizomatous. Culms 60-120 cm, terete. Sheaths terete; ligules 0.2-0.8 mm. Rames 6.5-12.5 cm, often purple. Sessile spikelets 5-5.5 mm; lower glumes with circular pits on the sides, the central region initially smooth, usually developing rectangular pits at maturity, occasionally remaining smooth; upper lemmas and paleas 4-4.5 mm. Pedicellate spikelets 1-2 mm. Caryopses about 2.2 mm. 2n = 18.

Coelorachis cylindrica is native to the southeastern United States, where it grows in tallgrass prairies, the edges of forests, and roadsides. The specimen from Michigan was found in an old field, in association with many native species. Its source is unknown.

2.   Coelorachis tessellata (Steud.) Nash
Pitted Jointgrass

Plants cespitose. Culms 80-120 cm, compressed-keeled. Sheaths compressed-keeled; blades to 41 cm long, to 7.8 mm wide, folded to flat, scabrous above. Rames 4.5-7(12) cm. Sessile spikelets 3.9-6.2 mm long, 2.1-2.4 mm wide; lower glumes with rectangular pits, keels narrowly winged distally. Pedicellate spikelets 2.3-2.7 mm, reduced to scales. 2n = unknown.

Coelorachis tessellata is endemic to the southern coastal plain of the United States, extending from Louisiana to northern Florida, although it is rare in Florida. It grows in bogs and moist pine woods, especially flatwoods.

3.   Coelorachis rugosa (Nutt.) Nash
Wrinkled Jointgrass

Plants cespitose. Culms 60-120 cm, compressed-keeled. Sheaths compressed-keeled; ligules 0.5-1 mm. Rames 3-9.5 cm; rachises distinctly indented adjacent to the sessile spikelets. Sessile spikelets 3-4 mm; lower glumes transversely rugose; upper lemmas and paleas 2-3 mm. Pedicellate spikelets 1-3 mm. Caryopses about 2 mm, broadly ellipsoid. 2n = unknown.

Coelorachis rugosa is endemic to the southeastern United States. It grows in moist to wet areas in prairies, bogs, and pine woods, especially flatwoods and savannahs.

4.   Coelorachis tuberculosa (Nash) Nash
Smooth Jointgrass

Plants cespitose. Culms 60-120 cm, compressed-keeled. Sheaths compressed-keeled, glabrous; blades to 31 cm long, to 7.8 mm wide, folded or flat, glabrous. Rames 4-8 cm; rachises not or only slightly indented adjacent to the sessile spikelets. Sessile spikelets 3.3-4.3 mm long, 1.3-2 mm wide; lower glumes smooth or sparsely and shallowly transversely rugose, keels narrowly winged. Pedicellate spikelets 1.9-2.6 mm, reduced to scales. 2n = unknown.

Coelorachis tuberculosa is an uncommon species, endemic to the southeastern United States. It grows in moist to wet areas such as bogs and pine woods, especially flatwoods and savannahs. s