Xylem carries water up from the roots which is against gravity. GIVE REASON? 

Dear Student, 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. The major mechanisms involved in the transportation of water through xylem in plants includes 1. Transpiration pull The major mechanism for long-distance water transport in plants is explained by the cohesion-tension theory, where the driving force of water transport is transpiration, i.e., the evaporation of water from the leaf surfaces. Water molecules cohere (stick together), and are pulled up the plant by the tension, or pulling force, exerted by evaporation at the leaf surface. Majority of water is transported through transpiration pull. Root Pressure Positive pressure created inside the xylem when water follows the ions transported along the concentration gradients into the vascular system .This can be observed when a freshly cut root stump continues to exude sap from its xylem vessels. The mechanism probably depends on active secretion of salts or other solutes into the xylem sap, thus lowering its water potential. Water then moves into the xylem by osmosis from neighboring root cells. Root pressure can transport water and dissolved mineral nutrients from roots through the xylem to the tops of relatively short plants when transpiration is low. Regards