Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Sep 4, green; pet numerous, white or pink, inserted on the ovary; stamens numerous, inserted on the ovary; inner stamens with linear filament and long anther, the outer with progressively widened, petal-like filament and shorter anther; stigmatic disk with 6–30 radiating lines; lf-blades floating, rotund or elliptic with a basal sinus; petioles and peduncles with 4 large air-passages. (Castalia) 30, widespread.