Polar water molecules adhere to the walls of xylem tracheids and vessels and cohere to each other which allows an overall tension and form ‘columns’ of water in the plant. The columns of water move from root to shoot and the water content of the soil supplies the ‘columns’ with water that enters the roots via osmosis. The difference between the water potentials of the soil and the air around the stomata are capable of producing enough force to transport water through the plant—from bottom to top and thus goes the cycle.
Roughly 90% of the water that enters a plant is lost via transpiration. Transpiration is the loss of water vapor through the leaves, just to refresh you. In addition, less than 5% of the water entering the plant is lost through the cuticle. Water is vital to plant life, not just for turgor pressure reasons, but much of the cellular activities occur in the presence of water molecules and the internal temperature of the plant is regulated by water. Recall that the xylem pathways go from the smallest part of the youngest roots all the way up the plant and out to the tip of the smallest and newest leaf. This internal plumbing system, paired with phloem and its nutrient transportation system, maintains the water needs and resources in the plant. The issue of the processes by which water is raised through columns—of considerable height at times—has been studied and debated for years in botany circles. The end result is the cohesion-tension theory.