AQUATIC PLANT STEMS
The water lily, Salvia and Wolfia whose stems remain in water are permeable to air and water. Oxygen dissolved in the water diffuses through the stem into the cells and carbon (IV) oxide diffuses out into the water.
Gaseous Exchange in Roots
Gaseous exchange occurs in the root hair of young terrestrial plants. Oxygen in the air spaces in the soil dissolves in the film of moisture surrounding soil particles and diffuses into the root hair along a concentration gradient. It diffuses from root hair cells into the cortex where it is used for respiration. Carbon (IV) oxide diffuses in the opposite direction. In older roots of woody plants, gaseous exchange takes place through lenticels.
Roots of aquatic plants e.g. water lily are permeable to water and gases. Oxygen from the water diffuses into roots along a concentration gradient. Carbon (IV) oxide diffuses out of the roots and into the water. The roots have many small lateral branches to increase the surface area for gaseous exchange.
They have air spaces that help the plants to float. Mangroove plants grow in permanently waterlogged soils, muddy beaches and at estuaries. They have roots that project above the ground level.
These are known as breathing roots or pneumatophores. These have pores through which gaseous exchange takes place e.g. in Avicenia the tips of the roots have pores. Others have respiratory roots with large air spaces.