Why are there no root hairs on the roots of aquatic plants?

Root hairs are where most of the nutrients and water are absorbed. The root tip produces new cells on a daily basis, and therefore also root hairs. When a plant lacks water and / or nutrients, it will release more assimilates (energy from photosynthesis) to produce more cells at the root tip. When the environment becomes drier, the root starts looking for water and will produce more cells and therefore more root hairs. For this reason, aquatic plants do not need root hairs.