They are highly dissected or ribbon-like.
Explanation
Submerged aquatic plants (hydrophytes) live entirely underwater and face specific physical challenges, particularly the drag force exerted by water currents. To survive, these plants exhibit morphological adaptations that minimize resistance to water flow while ensuring structural integrity.
Option Analysis:
- They develop a thick waxy cuticle. is Incorrect: A thick waxy cuticle is primarily an adaptation in terrestrial and xerophytic plants to prevent water loss through transpiration. Submerged plants typically lack a cuticle or possess a very thin one, as there is no risk of desiccation and they absorb water and nutrients directly through their surface.
- They are highly dissected or ribbon-like. is Correct: The leaves of many submerged plants are highly dissected (finely divided) or ribbon-like (e.g., Vallisneria). This streamlined shape reduces the surface area exposed to the direct force of the current, allowing water to flow easily through or around the leaves without tearing them.
- They develop thorns for mechanical protection. is Incorrect: Thorns are mechanical adaptations used for protection against grazing or to reduce transpiration in dry environments. They do not aid in withstanding water currents.
- They are broad and float on the surface. is Incorrect: Broad leaves that float on the surface are characteristic of floating hydrophytes (e.g., Lotus, Water Lily). If submerged leaves were broad, the increased surface area would create significant drag against water currents, leading to physical damage.
Key Takeaway:
Submerged hydrophytes adapt to water currents by developing narrow, ribbon-like, or highly dissected leaves. This morphology minimizes physical resistance to flowing water, preventing mechanical damage to the plant.