What is the smallest of all flowering plants that appears as minute green grains floating on the water?

Watermeal is indeed the smallest of all flowering plants, characterized by its tiny, free-floating green grains that can often be seen on the surface of still waters. This plant belongs to the family Lemnaceae and is notable for its diminutive size, with individual plants often being less than 1 millimeter in diameter.

Watermeal's tiny structure allows it to thrive in aquatic environments where it can absorb nutrients directly from the water. Its presence can significantly impact aquatic ecosystems as it competes for resources with other plants and algae. In the case of dense growth, it can block sunlight from reaching submerged plants, influencing the overall health of the aquatic habitat.

In contrast, duckweed, while also a floating plant, is larger than watermeal and has a distinct footprint with visible leaf structure. Cattails are emergent plants with tall stems that grow along the edges of water bodies, and Elodea, or waterweed, is a submerged aquatic plant. These differences highlight why watermeal is uniquely classified as the smallest flowering plant among the given options.