As two readers correctly surmissed, the main subject of this photo is the aroid Pistia stratiotes, commonly known as water lettuce. While the specimens shown here appear to have become stranded, water lettuce grows as free-floating rosettes on the surface of open bodies of water. The leaves radiate from an extremely shortened central stem with the tiny flowers produced in pockets at the bases of the leaves. As well as producing flowers, water lettuce reproduces vegetatively by the production of lateral stolons that give rise to daughter rosettes. Given time, a colony of water lettuce can carpet an entire lake.
I said that this ID was a clue to the next Taxon of the Week. Pistia stratiotes is not the only floating member of the Araceae: the remaining examples will be the subject of the next post.