My December 11th post dealt with "duckweed." Seven genera were mentioned in that post. With Treebeard's invaluable assistance, I have done my best to identify them to species correctly.
For an excellent introduction to duckweeds and other interesting plants, may I direct you to Wayne's Word.
Sorry it took so long for me to get around to doing this, OW!
Azolla caroliniana (Mosquito fern or Water-velvet) This one is not a duckweed, but is often found along with the duckweed mats on the millpond. The Azolla is green in the summer but turns to a velvety red in the fall.
Ricciocarpus natans (a Thallose Liverwort commonly called Purple-fringed Riccia)
Wolffiella gladiata (Sword bogmat or Florida mudmidget)
Spirodela polyrrhiza (Common water-flaxseed, Big duckweed)
Spirodela punctata (Dotted water-flaxseed, Dotted duckweed, Dotted duckmeat) syn: Landoltia punctata
Lemna valdiviana (Pale duckweed)
Wolffia brasiliensis (Watermeal)