Water lettuce : A plant (Pistia stratiotes) which floats on tropical waters, and forms a rosette of spongy, wedge-shaped leaves.

Example:


Taxonomy
plants, Water lettuce, The leaves, aquatic plants, Non-Native plants, aquatic vegetation, Plant Identification, mats, Plant Index, pond owners, wildlife species, food value, algal, algae, Management Options, Pistia stratiotes, exception, Florida.

Defination from the web:
... Water lettuce is a free-floating plant with many spongy, dusty green simple leaves. ...