Mechanisms responsible for movement of water in plants:
- Root pressure:
  - It is a force generated in the roots of plants, contributing to the upward movement of water in the xylem.
  - It occurs due to the active uptake of water and minerals by root cells, leading to an increase in pressure within the xylem of the root.
  - Root pressure is particularly noticeable during periods of high soil moisture, when water absorption by roots is at its peak.
  - For example, in the morning, there is a high soil moisture level. Thus, water will enter the roots and accumulate in the plant, creating a slight rise in root pressure.
  - The rise in root pressure will force some water to exude through special leaf tip structures, forming drops.
- Capillary action:
  - After the water gets inside the xylem of the root, it moves to the xylem of the stem.
  - Once in the xylem of the stem, water is carried upwards by the second force at work, called capillary action.
  - Capillary action is the result of forces of molecular attraction between water molecules (cohesion force) and between water molecules and the walls of the xylem vessels (adhesion force).
  - There are molecular cohesion forces that link one water molecule to the other.
- Transpiration pull:
  - Transpiration from the leaves creates a water potential gradient, pulling water molecules upwards through the xylem vessels in a continuous stream, known as the transpiration stream.

Transpiration:
- It is the loss of water vapour by evaporation of water at the surfaces of mesophyll cells, followed by the loss of water vapour through the stomata.
- Water leaves the xylem vessels in the leaf where it forms a water film on the walls of mesophyll cells inside the leaf.
- The large network of air spaces between mesophyll cells acts like a highway for water vapour.
- With more surface area for evaporation, water vapour from the moist mesophyll cells can quickly diffuse into these air spaces and move towards the stomata for release.
- Transpiration predominantly occurs in land plants. Meanwhile, in submerged aquatic plants that lack stomata, transpiration is absent.

Wilting:
- Transpiration occurs as long as the stomata are open for the uptake of carbon dioxide for photosynthesis.
- The other function of transpiration is to provide evaporative cooling for the plant, similar to sweating in humans.
  - When water goes from a liquid to a vapour form that is released into the atmosphere, water molecules take energy from the surrounding, thus cooling the surroundings.
- However, the rate of water loss from transpiration should not exceed the rate of water absorbed from the soil.
- When the transpiration rate exceeds the water absorption rate, the plant cells become flaccid because they lose turgidity.
- This causes the plant to wilt.
- Wilting is a good way for a plant to avoid further water loss by reducing the leaf surface area from which evaporation can occur.

Factors affecting the rate of transpiration:
- Humidity, temperature, and wind velocity:
  - Dry air, high temperature, and high wind velocity increase the transpiration rate.
  - On the other hand, humid air, low temperature, and low wind velocity slow down the rate of transpiration.
- Light intensity:
  - This factor does not directly affect the rate of transpiration.
  - Light intensity affects the size of the stomatal opening through which the water vapour passes.
  - When a plant is put under a bright light, the stomata open widely, thus increasing the rate of transpiration.
  - On the contrary, when a plant is put under a dim light, the stomata open narrowly, resulting in a decreased rate of transpiration.
- Number of stomata in a leaf
  - It is an intrinsic factor that influences the transpiration rate.
  - Stomata are more abundant at the lower epidermis of a leaf, so the transpiration rate in the lower epidermis of a leaf is higher than the transpiration rate in the upper epidermis of a leaf.
Image of an experiment to investigate transpiration rates:

8.02 - Transpiration

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