Spirodela

Spirodela is a genus of aquatic plants, one of several genera containing plants commonly called duckweed. "Spirodela" species are members of the Araceae under the APG II system. They were formerly members of the Lemnaceae.
"Spirodela" species are free-floating thalli; two to five plants may remain connected to each other. Plants are green, but may have a red or brown underside. Multiple roots (seven to 12) emerge from each thallus. "Spirodela" is larger () than "Lemna" ( – , one root per thallus).
Certain species of "Spirodela" overwinter as turions, a dormant form that lacks air pockets, so sinks to the bottom of the pond. In spring, turions rise to the surface and germinate to start a new population.
"Spirodela" often forms floating mats with related species, e.g. "Lemna" and "Wolffia".
The genus is virtually cosmopolitan in distribution. "Spirodela punctata" is sometimes treated as "Landoltia punctata".