The circulation of water through the xylem vessels is achieved mainly through the transpiration pull. The driving force for this process is transpiration from the leaves. This is referred to as the cohesion-tension-transpiration pull model of water transport. Factors that influence transpiration are (1) Water is transient in plants. (2) Less than 1 per cent of the water reaching the leaves is used in photosynthesis and plant growth. (3) Most of it is lost through the stomata in the leaves. This water loss is known as transpiration. The transpiration pull helps in the upward intake of water through roots and circulation of water through the xylem vessels to all the parts of the plant.