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.

Aquatic herb, free-floating or forming a mat to 4 cm thick usually 5 - 10 mm across

Stem: prostrate, 0.5 - 1 cm long, hairless.

Leaves: alternate, stalkless, dark green (sometimes with red margins) or dark red, two-lobed. The upper (emersed) lobe is several cells thick with a hairy upper surface, the largest hairs near stem having two or more cells. Lower (immersed) lobe one cell thick except near base, a little larger than the upper lobe, cup-shaped.

Roots: 3 - 5 cm long, unbranched.

Fruit cases: (sporocarps) paired, borne at base of lateral branches, either containing male spores (microsporocarp) or female spores (megasporocarp). Plants are often infertile, lacking sporocarps.

Female cases: (megasporocarps) each bearing one functional female spore (megaspore) 0.2 - 0.6 mm across, covered with dense tangled hairs, having three floats covered by a dark cone-like flap (indusium), lacking raised angular bumps or pits.

Male cases: (microsporocarps) each bearing many male spores (microspores) 3 micrometers across, covered with arrow-like barbs.

Similar species: Azolla mexicana differs by having pitted megaspores that are sparsely covered with a few long, slender hairs. However, a scanning electron microscope is required to see the megaspore surface, and many herbarium specimens completely lack megaspores. Therefore, many specimens cannot reliably be determined as one species or the other. Additionally, many taxonomists previously identified specimens based on the number of cross-walls in the barbs of microsporangial masses and on the general width of the plant, both being unreliable characteristics. Based on the expected distributions, the Chicago Region specimens are most likely A. caroliniana. See the Flora of North America link below for further detail on the taxonomic issues within this genus.

Habitat and ecology: Occasionally floating in stagnant or slow-moving waters.

Occurence in the Chicago region: native

Notes: Azolla species have a symbiotic relationship with Anabaena azollae, a blue-green algae which fixes atmospheric nitrogen. Anabaena azollae grows in stem tips, beneath the indusium, and in upper leaf lobe cavities. Therefore, some species of Azolla are harvested and used as fertilizers or nutritional supplements in livestock feed.

Etymology: Azolla comes from the Greek words azo, meaning "to dry," and ollya, meaning "to kill," referring to death caused from lack of water. Caroliniana means "from the Carolinas."

Author: The Morton Arboretum

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.

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