The xylem vessels of tall trees transport abundant water from the roots to the leaves situated a height of several meters. What is the force that causes this movement?
Water is the major constituent of plant tissues. The plant absorbs large amounts of water from the soil. Of the water absorbed from the soil, only a small proportion of it is retained by the plants while the rest of it is passed off into the atmosphere. So, transpiration can be defined as” the loss of water by evaporation from the aerial parts (cells) of the plant, especially through the stomata of leaves.” As a consequence of loss of water by transpiration, plants require large amounts of water for their survival.
Some of the water entering the leaves diffuses into the cells of the Mesophyll tissue of the leaves through their cellulose cell walls (as cellulose is freely permeable to water). The mesophyll cells are in contact with the air spaces in the leaf. The water from the cell of the mesophyll diffuses through their walls into the intercellular spaces from where it evaporates and diffuses into the atmosphere through the stomata. The evaporation of water from the top of the leaf reduces the concentration of water but increases the concentration of solutes in the leaf cells. So because of higher osmotic pressure in the leaf cells form the xylem. The uptake of water from the top of the xylem lowers the hydrostatic pressure at upper region of the xylem vessels. Whereas at base of the vessels, this pressure at the base of the vessels produces a pull or tension which pulls the water from the area of high pressure (lower xylem) to the area of low pressure ( upper xylem).
The pull or of suction force thus produced in the xylem vessels by the evaporation of water (transpiration) from the leaf is called transpiration pull. As a consequence of the transpiration pull and the cohesion and adhesion of water molecules, the water moves up the xylem vessels as an unbroken column called transpiration stream. The xylem vessels are a continuous system of tubes from the roots, to the stem and the leaves.