Answer:
(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.