Plants green or often blue-green to dark red, some red-fringed leaves usually present in nature, free-floating or forming a multilayer mat to 4 cm thick in early summer; plants frequently fertile. Stems prostrate, 1--1.5 cm. Largest hairs on upper leaf lobe near stem 2(--3)-celled; broad pedicel cell often 1/2 or more height of hair, apical cell curved, with tip nearly parallel to leaf surface. Megaspores not covered with raised angular bumps, pitted and sparsely covered with a few long filaments extending over surface. Azolla mexicana is generally less cold tolerant and has a narrower environmental range than A . caroliniana . Both species are closely related and are similar vegetatively in culture. In the western United States, A . mexicana is often fertile. Distribution in the Great Plains area is tentative and needs further study. In the eastern United States, A . mexicana may have been occasionally introduced.

Plants seldom fruiting, a little larger than no. 1 [Azolla caroliniana Willd.], mostly 10-15 mm wide, ±pinnately branched when well developed, dichotomous only at the periphery; upper lf-lobes 0.7-0.9 mm, not much smaller than the lower, ±strongly imbricate; microsporocarps mostly a little over 1 mm thick; massulae 0.2-0.3 mm; glochidia with several cross-walls; megaspores 0.4-0.5 mm, the rounded base strongly pitted. Floating in still water; widespread in w. U.S., s. to n. S. Amer., and e. to Wis., Ill., and Ark. Rare.

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

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