Plants green to yellowish green or dark red, with 2 growth stages; plants fertile only in mature stage, generally in late spring. Stems prostrate when immature, 1--3 cm, internodes elongate to 5 mm, becoming nearly erect to 5 cm or more when mature and crowded. Hairs on upper leaf lobes strictly unicellular. Megaspores warty with raised angular bumps, each with a tangle of filaments.

Azolla filiculoides is cold tolerant, surviving even in fragmented parts under thin ice. It usually reaches a climax population in late spring, becomes fertile, collapses, and is replaced by other more heat-tolerant aquatics such as Lemna spp. Hybrids between this species (male) and A . microphylla Kaulfuss (female), a species of Central America, South America, and the West Indies, have been reported (Do V. C. et al. 1989). V. M. Bates and E. T. Browne (1981) reported A . filiculoides from Georgia, far removed from its main range in western North America. The most likely explanation is that the plants represent escapes from horticulture.

PLANT: 0.7-3.0 cm long, sometimes to 5 cm in cultivation (Fig. 3). STEMS: glabrous. FLOATING LEAF LOBES: 0.5-0.8 mm long, somewhat convex, usually with hyaline or reddish margins (turning entirely dull reddish brown in the autumn), the minute adaxial trichomes 1-celled. MEGASPORES with the hemispherical portion having irregularly confluent, angular papillae, sometimes appearing irregularly pitted, also sparsely to moderately tomentose with long, loosely curled trichomes; the collar somewhat concave, glabrous, the cap appearing fibrous. GLOCHIDIA: with an average of 3-5 crosswalls. 2n = 44, 66. NOTES: Ponds, lakes, and backwaters of rivers, in still or sluggish water, sometimes stranded on mud: Cochise, Coconino, Graham, Greenlee, La Paz, Maricopa, Mohave, Pima, Pinal, Santa Cruz cos. (Fig. 1; also reported, but not vouchered, from Yuma Co.); 600-1200 m (2000-4000 ft); WA to CA and AZ, w Can., Mex., s to S. Amer.; introduced in HI and the Old World.Populations of A. filiculoides in Arizona are uncommon and sporadic. Waterfowl move the plants around and eventually it may be discovered at other sites in the state. REFERENCES: G.Yatskievych and M.D. Windham, 2008, Vascular Plants of Arizona: Azollaceae. CANOTIA 4 (2): 31-34.