Water is a very important substance obtained and transported throughout plants Transpiration stream is the movement of water through a plant. Transpiration stream is maintained by: 1. Osmosis 2. Root pressure 3. Transpiration Osmosis is the movement of water from high water concentration to low water concentration across a semi-permeable membrane. Root pressure is the force exerted by water within the xylem tissue of the roots. Transpiration is the loss of water from the aerial parts of a plant. Water and mineral uptake: Water enters the root hairs by osmosis, moving from high water concentration to low water concentration. Minerals dissolve easily in water and move into the root by diffusion - either by passive transport or by active transport (requires ATP). Water moves across the ground tissue and into xylem tissue. Water is then transported up the plant. Cohesion-tension model of water transport John Joly and Henry Dixon were two Irish scientists who first proposed the cohesion-tension model of water transport. Water moving into the xylem tissue of the root causes a pressure build up - this is called root pressure. Root pressure contributes to the upward movement of water molecules. Water molecules have hydrogen bonds between them maintaining them in the liquid form - this is cohesion of the water molecules. Water molecules also tend to stick easily to the sides of the xylem vessels - this is called adhesion of the water molecules. Transpiration of the water molecules occurs mainly from the leaves - this pulls the column of water molecules upwards through the xylem, creating a tension in the water molecules. Control of transpiration Transpiration is controlled by: Waxy cuticle - prevents direct water loss from the surface of leaves. Stomata - controls the rate of transpiration by opening and closing. Lenticels - allows a small amount of transpiration but also allows oxygen in (for respiration) and carbon dioxide out (excretion). .