Which is the smallest plant with root, stem and leaf ?

 Watermeal (Wolffia sp.) holds the unique position as the world's smallest flowering plant.

Watermeal is a member of the duckweed family and is approximately 1/42 inches long and 1/85 inches wide. It is as small as a broken rice or mustard grain. This plant is an aggressive invader of ponds and can also be dangerous for aquatic plants and fish in the pond.