Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cal-tube obconic, equaling or exceeding the triangular lobes; cor salverform, constricted at the throat; stamens included; style very short; stigma capitate; capsule 5-valved, enclosed in the persistent cal; small winter-annuals with a rosette of basal lvs and one to several scapes bearing terminal, involucrate umbels of very small fls. 100, N. Temp. and Arctic.