2. Azolla mexicana C. Presl (Mexican mosquito fern, water velvet) Pl. 15b,c; Map 10

Plants 0.7–2.0 cm long. Floating leaf-lobes 0.6–0.8 mm long, usually with hyaline or reddish margins, usually imbricate, usually densely papillose on the upper surface. Megaspores uncommon (but observed in some Missouri material), the surface with fibrous and pitted texture, with scattered, curly, long hairs. Glochidia with an average of 2–4 crosswalls. 2n=44. June –October.

Scattered, mostly in counties bordering the Mississippi, Missouri, and St. Francis Rivers (U.S., Mexico, Central and South America). Floating on water in ponds, sloughs, swamps, ditches, marshes, and still backwaters of rivers and streams; also stranded on mud by receding waters.