Azolla caroliniana Willd.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Species Description - 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.