Tennessee's Reelfoot National Wildlife Refuge. Duckweed, the world's smallest flowering plant, grows on the surface of still or slow-moving water. It grows rapidly—sometimes too rapidly, occasionally covering whole lakes or drought-slowed rivers—and provides protection for water creatures, control of excess minerals, and a barrier against evaporation.

(Text adapted from and photograph shot on assignment for, but not published in, the National Geographic book Wildlands for Wildlife, 1976)