Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Stems branched and jointed, the joints cylindric to flattened; spines and glochids arising from the areoles, or the plants virtually spineless; fls borne within the areoles near the tips of joints of the previous year; pet and sep rotate from the summit of the scarcely prolonged hypanthium; stamens shorter than the pet; seeds wingless. 150+, New World.