Since aquatic plants live in water, they do not have to worry about drying out. But they have other problems. How can their roots obtain oxygen when they are anchored in the anaerobic (lacking oxygen) mud at the bottom of ponds and lakes? Various adaptations help aquatic plants solve this and other problems.
Pockets of air
Many aquatic species have pockets of air embedded in the tissues of their leaves and stems. Since air is considerably lighter than water, this adaptation enables the leaves of deepwater aquatic plants to remain on the surface of the water. At the surface they receive more sunlight for photosynthesis (making food with the help of the sun's energy), and they can get the gases that they need for photosynthesis and respiration from the atmosphere.
In some cases, these air pockets are an avenue to provide oxygen to the roots. Roots normally get oxygen for respiration from pockets of air in the soil. But the mud on the bottom of lakes and ponds contain little if any oxygen. The air pockets in the tissues of the yellow water lily provide an avenue for currents of air. Oxygen rich air enters the stomata (small air holes) of the leaves and slowly but steadily makes its way down to the roots. Meanwhile, air laden with carbon dioxide flows up to the leaves and exits.
Many aquatic plants keep afloat by trapping air bubbles in little hairs that occur on their leaves or stems. Other aquatic plants, like Nymphaea odorata, have large flat leaves that stay on top of the water by taking advantage of surface tension (the sticking together of water molecules so that flat objects do not easily break through the water surface).
Leaves that grow underwater tend to have a feathery or highly lobed form. This allows water to move freely through the leaves without damaging them. It also gives the leaves a greater surface area for absorbing water and minerals from the water.
While bushes and trees need strong stems for support, water provides support for plants that grow in deep water. This allows water to pass by the stem easily without damaging it. Shallow water plants like rushes have stems that are flexible enough to bend with the wind without breaking.
Plants that grow in deeper water have slender roots, the chief function of which is to anchor the plant in the sediment. They often lack the root hairs requisite for water and mineral uptake because other parts of the plant perform these functions. Roots of many floating plants are thin hairs that obtain nutrients for the plant from the water, but the swollen bladderwort lacks roots altogether. Bladderworts obtain nourishment by trapping and digesting insects in many little traps called bladders.
A winter adaptation
Spirodela polyrrhiza and some other floating plants form turions in fall. The turion is a dormant stage of the plant. It lacks air pockets, so that it sinks to the bottom, where it will not be harmed by the ice forming on the surface. In spring it will develop into a normal plant.