Define root pressure.

Root pressure is a phenomenon observed in plants where positive pressure is generated in the roots, leading to the upward movement of water and dissolved nutrients from the roots to the rest of the plant. This pressure is primarily a result of osmotic pressure within the root cells, which is created when minerals and other solutes are actively transported into the root xylem from the surrounding soil. As a consequence, water follows these solutes into the xylem by osmosis, creating a positive pressure that pushes the water column upwards through the plant.

Root pressure is most noticeable at night or during periods when transpiration (the loss of water vapor from the aerial parts of the plant) is low. It can be evidenced by the exudation of water droplets from the edges of leaves, a phenomenon known as guttation. However, root pressure alone is not sufficient to account for the movement of water to the tops of tall trees; it works in conjunction with other mechanisms like capillary action and transpiration pull.