(a) Plants continuously absorb water through their roots. This water is sent up through the stem to all parts of the plant, including the leaves. Only a little amount, of water is retained in the plant or utilised by it in photosynthesis. The rest of it gets evaporated into the atmosphere as water vapour through the stomata present in the epidermis of the leaves and other aerial parts of the plant. This creates a suction pressure which pulls up water from xylem of the roots to the stem and then to the leaves.
(b) Xylem tissues are in the form of capillary tubes (tracheids and fibres) where narrower the diameter, greater will be the force. Whenever the xylem vessels lay empty, such as during the loss of water by transpiration, the water from below rises into them by a capillary force.
During day time, water is lost from the surface of the leaves by the process of transpiration. In this process, more and more water molecules are pulled up due to their tendency of rentainingjoined (cohesion). Such pulling force created by the leaves is very important in the case of tall trees where an upward conduction of water takes place.
