the answer is -
loating plants are of two types: those which are rooted with floating leaves (e.g. Water Lily) and those which are not rooted in the sediment, but just float on the surface (e.g. Duckweed). Floating leaves are generally tough because they have to withstand the weather and water movement.
The green pigment-containing chloroplasts important for photosynthesis are restricted to the upper surface of the leaves which are the only surface to be well lit. Stomata (breathing pores), through which gas exchange takes place in the leaf, are also found only on the upper surface of the leaf. This upper surface often has a thick waxy cuticle to repel water and help to keep the stomata open and clear. Air-filled internal cavities are also often present.
Terrestrial plants such as trees have to develop an enormous quantity of structural material in order to rise above all the other plants and collect the lion's share of the light available. Water lilies provide a neat example of a plant which has managed to do exactly the same thing, but with the minimum of structural material. Weak stems produce a massive floating canopy of leaves which dominate the local aquatic plant community just as effectively as trees dominate in a woodland. The difference lies in their external medium. Water provides all the necessary support, whereas air does not.
