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.

General: Aquatic annual or perennial ferns, generally fan-shaped, free-floating or forming small, often multi-layer mats 1-5 cm thick, occasionally stranded on muddy banks, plants green or often blue-green to dark red, some red-fringed leaves usually present in na Leaves: Sessile, alternate, often imbricate, in 2 ranks along the upper side of the stems, 0.6-2 mm wide, each leaf with 2 lobes, the upper (emersed) lobes greenish or reddish and photosynthetic, with a narrow colorless margin, several cells thick, bearing a colony of blue-green algae (Anabaena) in an ovoid cavity at the base of the lower side, lower lobes often floating or immersed, slightly larger than upper lobes, mostly not green (often colorless and translucent), 1 cell thick except at the base and generally cup-shaped, the largest hairs on the upper leaf lobes near the stem 2-3 celled, with broad pedicel cells often half as much or more than the height of the hair, apical cells curved, with the tips nearly parallel to the leaf surfaces. Sporangia: Plants heterosporous (producing 2 kinds of spores), proliferous by axillary fragmentation, frequently fertile, sporocarps occurring in pairs or tetrads at the base of lateral branches or terminal on leaf lobes, members of pair of same sex or of different Ecology: Found in ponds or slow streams, or on muddy banks, to 4,000 ft (1219 m). Notes: These small, feathery auqatic ferns seem to float on the water's surface, the leaves somewhat tirangular, thickish, and succulent-looking. Vascular Plants of the Gila Wilderness notes,"It bears two kinds of spore bearing structures, called sporocarps. The male sporocarp is much larger and contains many sporangia, which in turn contain many spores. The smaller female sporocarp contains only one spore. Azolla mexicana can also grow in streamside mud. Each leaf has two lobes, an upper green or red photosynthetic lobe that has a colorless margin, and a lower translucent cup shaped lobe. The upper lobe has a chamber within which Anabaena azollae, a nitrogen fixing cyanobacterium (blue-green alga) grows." FNA notes, "The species of Azolla are difficult taxonomically because (1) about 80% of the specimens lack sori, which are necessary for identification, and (2) the characteristics needed to identify the species are difficult to observe. A scanning electron microscope is needed to see sculpturing of the megaspores, and a light microscope is needed to see the number of cells per hair on the upper leaf lobe. (These hairs are best seen in profile on mature leaves; at least 40_ magnification is needed.)". Etymology: Azolla comes from the Greek azo, to dry, and ollumi or olluo, "to kill, destroy," in reference to the manner in which these plants die in dry conditions, while mexicana means of or from Mexico. Sources: FNA 1993, Jepson 2012, Kearney and Peebles 1969, Vascular Plants of the Gila Wilderness online, http://www.wnmu.edu/academic/nspages/gilaflora/azolla_mexicana.html