Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Delicate, moss-like plants, subdichotomously branched above every third lf; roots slender, simple, inconspicuous; lvs alternate, minute, sessile, unequally bilobed, one lobe submersed and serving as a float, the other smaller, emersed, and containing a colony of a symbiotic blue-green alga, Anabaena azollae; sporocarps borne on the first leaf of a branch, paired at the tip of a short, bipartite secondary lobe originating from the submersed lobe near its base, appearing almost axillary; microspores 32 or 64, aggregated into 4 or more massulae, each massula with its own thin wall, which bears numerous slender appendages (glochidia) with terminal, retrorse hooks; megaspore with a hemispheric or broadly bell-shaped basal part and a sharply marked, complex, conical terminal part that eventually shows 3 basally spreading, apically confluent, longitudinal valves. 6, cosmop.