1) Freshwater plant 2) Genus wolffia 3) Liliopsid genus 4) Monocot genus 
(Rootless Duckweed, Water Meal) These plants are the smallest plants in the world. They look like green grains floating in the water. The most common is W. arrhiza, Water Meal or Rootless Duckweed. It is native to Africa, Brazil, Western Asia and Europe. This plant isn't grown in the water garden purposely, but can sometimes be found in portions of...
[n] - minute rootless aquatic herbs having globular fronds floating on or near the water surface and bearing one flower per frond
genus Wolffia noun minute rootless aquatic herbs having globular fronds floating on or near the water surface and bearing one flower per frond