Question

Which of the following maintains the continuity of the water column in xylem?
A. Presence of air bubbles
B. Cohesive property of water
C. Evaporation power of water
D. None of the above

Answer 1

The most accepted theory of Dixon and Jolly for the ascent of sap determines the maintenance of continuous water columns in xylem.

Complete answer:
According to Dixon and Jolly, the ascent of the sap is responsible for the occurrence of passive processes in plants. The ascent of sap takes place due to the transpiration pull which is generated due to the loss of water from the aerial parts of the plant.
A continuous water column supply is also required for the rise of water or ascent of sap.
The presence of the air bubble will interrupt the continuous water channel, thereby depriving the aerial parts of water.
Evaporation power of water from the external surfaces of the plant is responsible for creating a deficiency of water in the aerial parts but this process is very slow. It creates a negative potential of water, which in turn creates a transpiration pull which is not that significant for the ascent of sap.
The force between the two water molecules is called the cohesive force. Adhesive force is the force between the water molecule and the xylem. The cohesive force is responsible for keeping the water molecules together and thus maintaining continuity of the water column.
So, the correct answer is “Option B”.

Note:
It is not that evaporation power of water from the terminal surfaces will not show any effect in the creation of transpiration pull, but it is very negligible to be visible for creating a prominent effect.
Also, evaporation can only create a vacancy, but it is not responsible for the movement of water in a continuous pattern because the vacancy can create an air bubble which will disrupt the movement. The movement of water is possible only due to the cohesive forces between the water molecules which keeps them together.
Therefore the maintenance of continuous supply of water in the xylem is due to cohesion forces and transpiration pull.