In roots, the most efficient region of water absorption is the root hair zone.' Root hairs function as tiny osmotic 'systems. Each root hair has a thin 'permeable cell wall, a semi-permeable cytoplasm and an osmotically active cell . sap present in the central vacuole. Because of the latter, a root hair cell has a water potential of -3 to -8 bars. Water potential of the soil water is - 0.1 to 0.3 bars. As a result water of the soil passes into the root hair cell. However, water does not pass into its vacuole. Instead it passes into apoplast and symplast of cortical, endodermal and pericycle cells and enter the xylem channels passively because of the very low water. potential due to . tension under which water is present in them, caused by transpiration in the aerial parts. A gradient of water potential exists between root hair cell, cortical cell, endodermal, pericycle' and xylem channel so that flow of water is not interrupted.