Biology XI: 11 Transport in plants(5 Water Movement Up In a Plant)

Water Movement Up In a Plant As the water is absorbed by the roots from the soil and moved to the vascular system, it has to be transported to various parts of the plant. Two forces responsible for transporting the water up in a plant are root pressure and transpiration pull. Root Pressure Positive pressure created inside the xylem when water follows the ions transported along the concentration gradients into the vascular system Guttation − Loss of water in its liquid phase from special openings near tip of grass blades and leaves of herbaceous plantsThis is an effect of root pressure and is observable at night and early morning when rate of transpiration is low. Does not account for majority of water transportIt works to re-form the broken chains of water molecules in xylem that may break under enormous tension created by transpiration. Majority of water is transported through transpiration pull. Transpiration pull Pull of water as a result of tension created by transpiration is the major driving force of water movement upwards in a plant. (cohesion − tension transpiration pull) Transpiration accounts for loss of 99% of water taken by the plant. Loss is mainly through stomata. 3 physical properties of water affect the ascent of xylem sap due to transpiration pull. Cohesion − Mutual attraction between water molecules Adhesion − Attraction of water molecules to polar surface (xylem cell wall surfaces) Surface tension − Attraction of water to each other in liquid phase to a greater extent than to water in gaseous phase These properties give water high tensile strength and high capillarity. Tensile strength − Ability to resist pulley force Capillarity − Ability to rise in thin tubes (Aided by small diameter of tracheary elements such as tracheids and vessel elements)