Watermeal is closely related to Duckweed. They are different Genera in the same Family. Since they are both tiny floating plants that will sometimes cover the surface of a pond, they are often mistaken for each other. However, they can be distinguished by a few small details. Watermeal is smaller than Duckweed, often several times smaller. From a distance (standing height looking down) Duckweed often have visible individual leaves while Watermeal will simply appear to be a green paste. The leaves are only visible on closer examination. Another detail is that Duckweed will have roots dangling down into the water while Watermeal will not. It is simply the floating leaf and nothing else.
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