Hydrophytes

Hydrophytes

Hydrophytes are plants that live in or on water, spending all or most of their life cycle either fully or partly submerged, floating on the water surface, or rooted in waterlogged soil. Because water is never in short supply for them, their structure is shaped almost entirely around problems that land plants never face — staying buoyant, exchanging gases without free access to air, and surviving in a medium that offers no rigid support.

Kinds of hydrophytes

Hydrophytes are commonly grouped into five kinds based on their relationship to the water surface and the substratum:

• Free-floating hydrophytes — these float freely on the water surface with their roots hanging loose in the water, never anchored to the bottom. Examples: Pistia (water lettuce) and Eichhornia (water hyacinth).
• Rooted hydrophytes with floating leaves — these are anchored in the mud at the bottom by their roots, but send up long leaf-stalks so that the leaf blades themselves float flat on the surface. Example: Nymphaea (water lily).
• Submerged and suspended hydrophytes — these float freely below the water surface, entirely submerged, without being rooted in the substratum. Examples: Ceratophyllum and Utricularia.
• Submerged and rooted hydrophytes — these are anchored by roots in the mud at the bottom, but their entire shoot system remains submerged underwater. Examples: Hydrilla and Vallisneria.
• Amphibious hydrophytes — these grow partly in water and partly emerge above it or onto land, tolerating both aquatic and terrestrial conditions at different parts of the same plant or at different times. Example: Alternanthera.

Morphological adaptations

Because water is always available and buoyant, hydrophytes show several visible external adaptations: the root system is greatly reduced or altogether absent, since minerals and water can be absorbed directly through the submerged stem and leaf surfaces rather than needing an extensive root network. The cuticle — the waxy waterproof layer that land plants use to stop water loss — is very thin or missing entirely, because there is no risk of drying out underwater. Submerged leaves are typically thin, ribbon-like, or finely dissected into narrow segments; this increases the surface area available for gas exchange directly with the surrounding water and also lets the leaves bend with water currents instead of resisting and breaking under them. Floating leaves, by contrast, are usually broad and flat, and carry their stomata restricted to the upper surface only, since that is the only surface exposed to air. Many hydrophytes also develop air-filled tissue that gives the plant buoyancy, and some surfaces are covered in mucilage (a slimy coating) that offers protection from decay and from small aquatic organisms.

Anatomical adaptations

Internally, the most distinctive feature of a hydrophyte is a well-developed system of large air spaces running through the cortex and other tissues, called aerenchyma. This spongy tissue serves two purposes at once: it makes the plant buoyant enough to float or to stay upright in water, and it allows oxygen and carbon dioxide produced or needed by submerged tissues to diffuse internally between organs, compensating for the very limited direct exchange of gases with the open atmosphere underwater.

Hydrophytes also show markedly reduced xylem. In an ordinary land plant, xylem exists mainly to draw up water and dissolved minerals from the roots and carry them to every other part of the plant. A submerged hydrophyte does not need this long-distance transport system, because every submerged cell is already bathed in water and can absorb water and dissolved minerals directly across its own surface. With this demand removed, the xylem tissue that would otherwise be extensively developed is instead reduced to a minimum. …