Question

The rate of transpiration ……… the process of the ascent of sap
(a) Increases
(b) Decreases
(c) Do not affect
(d) Increases and then decreases

Answer 1

Hint: Transpiration creates a negative pressure which creates a pull in the upward direction and it aids the process of upward movement of minerals and water from the root to the leaves.

Complete Step by Step Answer:
As you may know that transpiration is the phenomenon of the loss of water through the leaves and which is lost to the atmosphere. This is the main reason behind the movement of the water present inside the xylem. Due to transpiration, a negative pressure also termed as tension, is created on the surface of the leaves. The pressure created is quite large and can be approximated to a value of - 2MegaPascals.

The rate of transpiration affects the pull or the negative pressure created. The rate of transpiration is further affected by the relative humidity in the weather which may vary according to the temperature conditions. Water or sap is pulled up from the roots by this pressure created through transpiration. And it will also be clear to you by now that the ascent of sap is directly proportional to the rate of transpiration. Hence, as transpiration increases the ascent of sap also increases.

Additional Information:
- Transpiration occurs through the tiny pores present on the leaf surface called the stomata.
- During the night, the stomata remain shut, the transpiration is also interrupted, due to which the ascent of sap stops too. The sap is held through the force of cohesion of the sap molecules in the cell walls present in the xylem vessels.
- The force or the tension created during the day when the stomata are open, which is used to pull up the sap to the upper extremities of the pant body combined with the cohesive force during the night with which the sap does not descend forms the cohesion- tension theory related to the ascent of sap.
So, the correct option is, “Increases”.

Note:
- You must know that the plants use the properties of water, i.e., the cohesive and the adhesive properties of water to successfully transport the sap to the upper parts.
- You must also note that even though the tension force is quite large, yet tension force may reach higher values depending on the height of the plant. For tall plants, the tension force required would also increase.