The rolled leaf and hairs both serve to trap moist air so reducing transpiration. In addition, a smaller surface area of leaf is exposed to the drying effects of the wind. Many xerophytic plants such as pine and heather, have small, needle shaped leaves which are often circular in cross section. This reduces the surface area and hence the evaporating surface. Hydrophytic plants, in contrast to xerophytes are adapted to living in freshwater or very wet habitats.
Water availability is therefore no problem.
The greatest problem faced by hydrophytes is how to obtain sufficient oxygen. Water logged soils have few, if any, air spaces, and what little oxygen there is, may be used in bacterial decay. In addition, oxygen has low solubility in water. These air filled spaces give the plant buoyancy, allowing the leaf to float on or near the surface and so maximize light capture. Oxygen produced by photosynthesis during the day is trapped in the spaces and can diffuse to other parts of the plant, enabling respiration.