The world's smallest flowering plants

Each green dot is a separate plant.

Duckweeds are the minute plant that can be found growing in large numbers on many still bodies of water. The main body of the plant is a flattened, often oval thallus (not a leaf but rather a highly reduced and fused leaf and stem). In three of the recognised genera of duckweeds, Lemna, Spirodela and Landoltia (the 'Lemneae'), one or more short roots emerge from one end of the thallus (the proximal end). In the other two genera, Wolffia and Wolffiella (the Wolffieae), the thallus lack roots...

The entire plant is generally less than five millimetres (the size reached by Landoltia punctata) while Wolffia individuals are less than half a millimetre long when mature...

The flowers of duckweed are correspondingly tiny and many species produce them only rarely...

More details at Catalogue of Organisms, via Fresh Photons.