With the increase of diffusion pressure deficit, these cells absorb water from adjoining cells, ultimately the water is
absorbed from xylem elements of vascular bundles of leaf.
Since the xylem elements are filled with continuous water column, a tension or pull called transpiration pull develops
at the top of the column. This tension or pull is transmitted down from petiole to stem and finally to roots leading to
upward movement of water.
Cohesion and adhesion of water in xylem
Xylem tracheids and trachea are long tubular structures filled with water, extending from root to leaf. Thus, one end of
xylem (continuous with one another) is in the root and other end is in the leaf. Water molecules remain attached to one
another by a strong mutual force of attraction called cohesive force, which is due to the presence of hydrogen bonds
amongst adjacent water molecules.
Supplementing the cohesion between water molecules is adhesion between water molecules and the walls of tracheary
elements of xylem. Thus, according to this theory, water ascends in the plant because of transpiration pull and this column
of water remains continuous because of cohesive and adhesive forces of water molecules.
TRANSpIRATION
The loss of water in the form of vapours from the living tissues of aerial parts of the plant is termed as transpiration.
The loss of water due to transpiration is quite high, 2 litres per day in sunflower, 36-45 litres in apple and upto 1 tonne per day
in elm tree. Approximately 98-99% of the water absorbed by a plant is lost in transpiration, 0.2% is used in photosynthesis
while the remaining is retained in the plant during growth.
Most of the transpiration occurs through foliar surface or surface of the leaves. It is known as foliar transpiration. Young
stems, flowers, fruits, etc., also transpire. Transpiration from stems is called cauline transpiration.
Different types of transpiration have been listed in the following flow chart.