Fully submerged plants are named hydrophytes and have a number of adaptations to allow them to live in water (Plant, 2001). If plants became submerged, they would need to adapt by reducing the presence of mechanical strengthening tissues of stems and leaf petioles, which would limit flexibility, and would be unnecessary because the plant would be supported by water. The external protective tissues land plants use to prevent water loss would also become unnecessary. All surface cells would be able to absorb water, nutrients and dissolved gases from the surrounding medium, so parts of the plants internal system of tubes (xylem) which normally transport water from the roots would not be needed. The root system would most likely be reduced, with their main function being anchorage. Root hairs, mainly used for absorption on land, would probably disappear.
The leaves of plants may have to adapt once the plant is submerged (Plant, 2001). Many submerged plant leaves are highly divided to create a large surface area for absorption and photosynthesis. This also diminishes water resistance and thus damage to the leaves from water movements. The leaves and stems of aquatic plants often have air-filled cavities which provide an internal atmosphere.
Marine grasses have long-flattened blade-like leaves, giving them a large surface area for photosynthesis, and a well-developed root system with rhizomes to store food (Greener, 2004). They are believed to have evolved from terrestrial plants which returned to marine life in gradual steps. The strategies for success for plants in a marine environment are to maximize sunlight exposure by staying in the photic zone and minimizing nutrient needs (Adaptions, 2004). Phytoplankton are the dominant marine plants because they achieve this by being small, simple plants, which are buoyant due to the oils they contain, and the fact that nutrients can diffuse in and wastes diffuse out readily....