Spirodela: Answers to key questions leading to this genus 

	Roots present, 1-21 per frond; fronds flat or gibbous, air spaces present and veins 1-21; daughter fronds and flowers arise in marginal pouches. NOT [Roots absent; fronds 3-dimensional (globular), without air spaces and without veins; daughter fronds arise from terminal pouches; flowers borne in dorsal cavities on upper frond surface.]
	Roots more than 1 per frond; fronds purplish brown below, obovate to circular, up to 10 mm long; veins visible,  7 or more.  NOT [Roots 1 per frond; fronds green or reddish below, usually narrower but if obovate, then 4 mm long at most; veins difficult to distinguish, usually 3.]

Lemnaceae: Answers to key questions leading to this family. 

Plants free-floating aquatics growing on the surface or suspended in water, solitary and 0.5-15 mm across or attached together in larger colonies; identifiable stems and leaves absent. NOT [Plants larger and with well-defined stems and/or leaves]