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and some seasons but it does not account for majority of water transport due to
which most plants fulfil their needs by transpiratory pull.
Transpiration Pull
Plants have a continuous water column in their xylem channels that starts at the
base, i.e., roots and continues up to leaves from where water is lost through the
process of transpiration. Thus, despite the absence of a circulatory system in plants,
the flow of water upward through the xylem in plants achieve fairly high rates up to
15 meters per hour.
The water molecules in the water column remain attracted by the cohesive force and
cannot be separated easily from one another. Thus, there is attraction between water
molecules and the inner wall of xylem ducts. Hence, water column cannot be pulled
away from the walls of xylem ducts due to strong adhesive and cohesive forces. This
maintains the continuity of water column from roots to leaves.
Water is lost from mesophyll cells to the inter cellular spaces as a result of
transpiration and develops a strong negative water potential. There are very large
number of leaves and each leaf has thousands of transpiring mesophyll cells, which
withdraw water from the xylem. This leads to a negative pressure in the water
column, which exerts an upward pull over the water column. This pull is known as
transpirational pull.
This tension or pull is transmitted up to the roots in search for more water. The water
column (formed in the xylem elements of roots) now moves upward under the
influence of transpirational pull.
Thus, the cohesive, adhesive forces and transpiration pull all help in lifting up of
water through xylem elements and because of the critical role of cohesion the
transpiration pull is also called cohesion-tension transpiration pull model of water
transport.
Note:
Cohesion-tension theory was originally proposed by Dixon and Jolly in 1894 and