Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Fls 4-merous; cal cleft nearly or quite to the base; cor campanulate, the lobes almost equaling the tube, imbricate in bud; stamens inserted at the sinuses of the cor; style very short and stout; stigma large, ± 2-lobed; slender, mycotrophic annuals or biennials, the lvs reduced to minute scales, the stem often spiral or even tending to twine, green to yellowish or purplish, prolonged into a slender terminal panicle (or in small plants a raceme) of small, white to purplish or yellowish fls. 3, e. N. Amer.