Q.Why do hydrophytes show reduced xylem?

Hydrophytes are plants adapted to living in or on water, and their whole structure is built around problems that never trouble an ordinary land plant — staying buoyant, absorbing water and minerals directly from their surroundings, and exchanging gases without free access to open air.

Classification

Hydrophytes fall into five recognised kinds based on how they are positioned relative to the water surface and the substratum beneath:

• Free-floating — drifting on the surface with roots hanging loose, never anchored to the bottom, as in Pistia and Eichhornia.
• Rooted with floating leaves — anchored in the mud below, but with leaf blades carried up on long stalks to float flat on the surface, as in Nymphaea.
• Submerged and suspended — floating entirely below the surface with no attachment to the substratum, as in Ceratophyllum and Utricularia.
• Submerged and rooted — anchored in the substratum by roots, but with the whole shoot remaining underwater, as in Hydrilla and Vallisneria.
• Amphibious — growing partly submerged and partly emergent above the water or onto land, as in Alternanthera.

Morphological adaptations

Because water and dissolved minerals are always available, the root system of a hydrophyte is greatly reduced or absent altogether, since absorption can happen directly across the submerged plant surface. The cuticle is thin or missing, since there is no risk of drying out. Submerged leaves are typically thin, ribbon-like, or finely dissected, maximising the surface available for direct gas exchange with the water and allowing the leaf to bend rather than resist water currents. Floating leaves, by contrast, are broad, with stomata confined to the upper surface that alone is exposed to air. Many hydrophytes also carry air-filled tissue for buoyancy and a mucilage covering for protection.

Anatomical adaptations …