1. Azolla caroliniana Willd. (eastern mosquito fern) Pl. 15a; Map 9

Plants 0.5–1(–1.2) cm long. Floating leaf-lobes 0.4–0.5 mm long, sometimes with thin margins, usually spreading and only slightly imbricate, nearly glabrous to somewhat papillose on the upper surface. Megaspores very rarely produced (not observed yet in Missouri material), the surface with fibrous texture, but not pitted, covered with a dense tangle of long hairs. Glochidia usually with no or 1 crosswall. 2n=44. June–October (in adjacent states).

Scattered in eastern Missouri (eastern U.S. and Canada, Mexico, West Indies, Central and South America; introduced in Europe, Asia). Floating on ponds or stranded on mud.

This species occurs primarily to the east of the state, and the Missouri distribution is supported by few, sterile specimens. Fertile collections would be desirable to confirm its presence here.