Plants dark green or with margins of bright crimson or whole plants dark red, free-floating or forming multilayer mat to 4 cm thick under good conditions; plants infrequently fertile. Stems prostrate, 0.5--1 cm. Largest hairs on upper leaf lobe near stem with 2 or more cells; broad pedicel cell often 1/2 or more height of hair, apical cell curved, with tip nearly parallel to leaf surface. Megaspores without raised angular bumps or pits, densely and uniformly covered with tangled filaments. The sporophyte of Azolla caroliniana commonly survives throughout the year in temperate areas (with hard frosts and prolonged ice cover). It is the best adapted of all species for subsistence on mud. Azolla caroliniana is rarely collected with sporocarps.
Plants rarely fruiting, minute, mostly 5-10 mm wide, dichotomously branched almost throughout; upper lf-lobes 0.5-0.6 mm, much smaller than the lower, not much imbricate; massulae ca 0.3 mm, the glochidia without cross-walls; megaspores unknown in U.S.; 2n=48. floating in still water; Mass. and N.Y. to Fla. and La. on the coastal plain, and up the Mississippi embayment to w. Ky. and (?) s. Ill.; W. Ind. Rare.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.