This theory was proposed by Dixon and Joly (1894) and has been supported by Curtis and Clark (1951), Levitt (1969).
The above theory is also konwn as Dixon theory of ascent of sap. The theory assumes that water is pulled from up, but not pushed from below. The theory has two essential features such as (i) cohesion of water and adhesion between water and xylem tiusses, (iii) Transpiration pull.
(i) Conhesion of water and adhesion between water and xylem tissues
Cohesion is the phenomenon of attraction between similar molecules. The water molecules remain attracted by the cohesive force and cannot be easily separated from one another. Further, there is attraction between water molecules and the inner wall of xylem ducts.
Then water column cannot be pulled away from the wall of xylem ducts due to strong adhesive and cohesive properties of water and the continuity of water column is maintained from roots to leaves.
The forces acting against the cohesive force of water and which try to break up the water column in the plant are the weight of water column itself, the resistance encountered by water in crossing the tissues of the root, the stem and the mesophyll cells of the leaf.
All these forces combined together have been found to be of the value of 50 atms. In tall trees. But the magnitude of the cohesive force of water alone is about 300 atms which is sufficient for maintaining the continuity of water column in plants
It is the pulling force responsible for lifting the water column. As water is lost in form of water vapour to atmosphere from the mesophyll cells by transpiration, a negative hydrostatic pressure is created in the mesophyll cells which in turn draw water from veins of the leaves.
The negative tension is then gradually transmitted downwards via xylem tissues of the leaf, stem and finally to the roots. As a result there is a continuous upward movement of water column in the plant. 1 atm. pressure can raise water to a height of more than 32ft. So a tension of 13 atm is needed to raise water to a height of 416 feet, scientist have measured this tension to be more than 75 atm. in case of trees, more than 400 feet in height.
Thus the transpiration pull acts as pull from above on the-whole of water column of the plant which pushes the water column of xylem vessels of roots lowers leaves i.e. in an upward direction. This is how ascent of sap is affected in plants.
The whole process can be compared to a person (transpiration pull) pulling a bucket full of water with a steel rope (unbroken and continuous water column)
Plants absorb a large quantity of Water, from soil by means of roots and root hairs. However, only a small fraction (1.2%) of the absorbed water is utilised by the plants for its metabolic activities.
The remainder (98-99%) excessive amount of absorbed water is lost to the atmosphere by the physiological process known an transpiration. It is a universal process. Transpiration is defined as the loss of water in the form of water vapour from the internal tissues of living plants through the aerial parts such as leaves, green shoots etc., in the presence of sunlight.
Stages in Transpiration
Transpiration occurs through two successive stages:
i. At first water is lost from the mesophyll cells of the leaf into the intercellular spaces.
ii. Subsequently water diffuses from the intercellular spaces into the outer atmosphere in the form of water vapour either through the stomata or general surface of epidermis of leaves.