Azolla  

The Azollacea are a very small free floating aquatic fern. The Azollacea has 1 genus with 6 species and is wide spread through tropical and sub-tropical regions. The stem of this particular fern is short. The plant grows in a creeping like fashion. 

figure 1.1 Azolla in water body.(USA: Texas: cultivated, 2004.Image at PlantSystematics.org)