Two major forces help in the transport of water in a plant. Force A is the driving force in the movement of water during the day, whereas force B helps the movement of water in a plant during the night or during the day when humidity is very high. (a) Identify force A and force B. (b) Describe how each of these forces helps in the movement of water in a plant.

(a)
Force A: Transpiration pull is the driving force in the movement of water during the day. Transpiration is the process by which water is evaporated from the stomata of plant leaves, creating a negative pressure that pulls water upward through the xylem vessels.

Force B: Root pressure is the force that helps the movement of water in a plant during the night or during the day when humidity is very high. Root pressure is the pressure exerted by the roots that forces water up the stem, even against gravity.

(b)
1. Transpiration Pull (Force A):
Description: Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems, and flowers.

Mechanism: Water is absorbed by the roots from the soil and transported up to the leaves through the xylem vessels. As water evaporates from the stomata on the surface of leaves, it creates a negative pressure or tension in the xylem, leading to a suction force known as transpiration pull. This force helps to pull water upward, against gravity, from the roots to the leaves. It also helps in the ascent of minerals and nutrients dissolved in water.

2. Root Pressure (Force B):

Description: Root pressure is the pressure developed in the roots due to active absorption of ions from the soil and their accumulation in the root cells.

Mechanism: During the night or when transpiration is low, the roots continue to actively absorb mineral ions from the soil. These ions accumulate in the root cells, creating a higher concentration of solutes. Water then moves into the roots from the soil to balance this increased solute concentration, creating positive pressure known as root pressure. This pressure helps to push water up the stem and into the xylem vessels, aiding in the movement of water. Root pressure is particularly significant in certain plant species and under conditions of high soil moisture or during the night when transpiration is reduced.

In summary, transpiration pull (force A) is the primary force responsible for water movement during the day, driven by the evaporation of water from the leaves. Root pressure (force B) contributes to water movement during the night or when humidity is high, driven by the active absorption of ions by the roots and the resulting increase in solute concentration.