Hydrophytes: 

Hydrophytes are plants that grow or live submerged in lakes , ponds , swamps and rivers . Examples of hydrophytes include water lilies and elodea . Hydrophytes have a shortage of oxygen and an excess of water , so they have special adaptations to allow oxygen and water to be used for photosynthesis and respiration . 

                                          

Hydrophytes which are fully submerged, such as elodea have very thin leaves to increase the surface area to volume ratio , which increases the rate of diffusion of minerals and gases (CO₂ and O₂) into the plant cells to be used for respiration and photosynthesis. Also hydrophytes that are fully submerged have no stomata or waxy cuticle as these are not required, instead stomata and the waxy cuticle are found on floating parts of the leaf to allow efficient gas exchange and to prevent excess water loss on very hot days, if the stomata and waxy cuticle were present on submerged leaves, they may be dense and therefore effect the buoyancy of the lea , sometimes a very thin cuticle is present on submerged leaves to prevent algae, but still allows photosynthesis . By having very thin leaves with no stomata and a thin waxy cuticle , or leaves with stomata and waxy cuticle only on the floating tissue , hydrophytes are able to maintain a water balance at which they are can diffuse minerals and gases and prevent excess transpiration .

Hydrophytes have very large air spaces around their leaf cells , especially the spongy and palisade (mesophyll) layers , this aids buoyancy in the leaves , so therefore they are closer to the surface for photosynthesis , it also facilitates diffusion of minerals and gases for photosynthesis and respiration. By having large air spaces around their leaf cells , hydrophytes are able to maintain a water balance at which minerals and gases can be diffused into the leaf , and also allows the leaf to float for optimum photosynthesis .

Hydrophytes usually have small roots because minerals and gases which are needed for respiration and photosynthesis are dissolved in the surrounding water , or diffused into the small thin leaves that are adapted specifically to allow diffusion , so therefore the roots are only needed for anchorage and stability , for example water caltrop  and giant water lilies .By having small roots hydrophytes are able to maintain a water balance which is optimum for successful photosynthesis and respiration , as the roots are not needed for diffusion of minerals and gases, so therefore they don't absorb water as this is in excess .

 

photos from:

http://www.flickr.com/photos/7294954@N02/485253237/

http://en.wikipedia.org/wiki/Elodea

http://ladyjskincare.blogspot.co.nz/2011/07/beauty-obsession-of-day-water-lilies.html