Water hyacinth has a special type of parenchyma tissue which is known as the aerenchyma.
Aerenchyma is the spongy tissue that has air spaces or channels where the air is trapped, especially in the stem of the hyacinth.
The stem part becomes buoyant and due to this, the water hyacinth floats on the surface of the water.
Aerenchyma helps in buoyancy as well as in the respiration of aquatic plants.
Cells of aerenchyma continue from shoots to roots that help in the diffusion of air, thus maintaining sufficient oxygen levels for respiration throughout the plant.
Hence, water hyacinth floats on the surface of the water.