Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Spikelets 1(2)-fld, not disarticulating, borne in triads on opposite sides of the rachis, the lateral spikelets often pedicellate and sterile, the central one sessile and fertile; glumes elongate, awned or awn-like, setaceous throughout or widened at base, the 6 in each triad of spikelets forming a false invol to the florets; lemma of the lateral spikelets often reduced or abortive, that of the central spikelet indurate, obscurely veined, its rounded back turned away from the rachis, usually long-awned; rachilla prolonged behind the palea as a short bristle; grasses with mostly flat lvs, scarious, truncate ligules, and dense, bristly spikes which, except in H. vulgare, disarticulate at each joint. (Critesion) 35, widespread.