Spirodela polyrhiza.de.wikipedia.org (Accessed 4/2014).

Spirodela polyrhiza is a perennial monocotyledon herb, from overwintering bulblets, colonial plants, without stems or leaves, a simple, free floating, disk-like body, (thallus) broadly oval or widest above middle, 0.5-0.8 cm long and almost as wide, thick, purple, convex below, dark green above, 5-11 palmate-veined, 5-15 tiny roots forming from underside of nodal point of body, reproducing by budding from nodal point. 

Flowers seldom produced, minute, in a tiny pouches and consisting of two anthers and separate ovary and style. 

Fruit 1-2 seeds. Bulblets produced mostly in autumn, sink to bottom of waterbody to overwinter, rising in spring to commence vegetative growth. 

Wetland status: OBL. 

Frequency in NYC: Very infrequent. 

Origin: Native. 

Habitat: Quite ponds.