Aquatic plants - also called hydrophytic plants or hydrophytes - are plants that have adapted to living in aquatic environments.
One of the main problems facing submerged aquatic plants is their inability to obtain oxygen. Unlike terrestrial plants, these plants cannot obtain the vital gas through their stomata because they are submerged in water.
Therefore, the stems, roots, and leaves of submerged aquatic plants posses aerenchyma cells, which supply oxygen to the rest of the plants.
Aerenchyma is a parenchyma tissue with large intercellular air spaces. It stores and transports oxygen to living tissues.
Air spaces within the tissues help to keep the aquatic plant buoyant so that its leaves can reach the top of the pond, thus maximising the amount of sunlight it receives.
Submerged aquatic plants utilise living in water to their fullest advantage. Since these plants are in no danger of drying out, the leaves have few or no cuticles on the surface of their leaves.
In addition, the stems of these plants are limp and delicate with little strengthening tissue because they utilise the water for support.
The leaves tend to be thin, flexible and narrow. These finely dissected leaves offer little resistance to running water and can be dragged through the water without tearing.
Characteristics of hydrophytes:
1. A thin cuticle. Cuticles primarily prevent water loss, thus most hydrophytes have no need for cuticles.
2. Stomata that are open most of time because water is abundant and therefore there is no need for it to be retained in the plant. This means that guard cells on the stomata are generally inactive.
3. An increased number of stomata, that can be on either side of leaves.
4. A less rigid structure: water pressure supports them.
5. Flat leaves on surface plants for floatation.
6. Air sacs for floatation.
7. Smaller roots: water can diffuse directly into leaves.
8. Feathery roots: no need to support the plant.
9. Specialized roots able to take in oxygen.
For example, some species of buttercup (genus Ranunculus) float slightly submerged in water; only the flowers extend above the water. Their leaves and roots are long and thin and almost hair-like; this helps spread the mass of the plant over a wide area, making it more buoyant. Long roots and thin leaves also provide a greater surface area for uptake of mineral solutes and oxygen.
Wide flat leaves in water lilies (family Nymphaeaceae) help distribute weight over a large area, thus helping them float near surface.
Many fish keepers keep aquatic plants in their tanks to control phytoplankton and moss by removing metabolites.
Many species of aquatic plant are invasive species in different parts of the world. Aquatic plants make particularly good weeds because they reproduce vegetatively from fragments.