11. Describe transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?
Transpiration pull model of water transport in plants
Transpiration is the loss of water in the form of water vapour from aerial parts of plants. In tall trees, the water rises with the help of the transpirational pull generated by transpiration. This is called the cohesion-tension model of water transport. According to this model, the water is mostly pulled from roots to leaves due to the driving force of transpiration from the leaves. The water molecules remain attached with one another by cohesion force. The water molecule does not break in vessels and tracheid due to adhesive force between their walls and water molecules. On account of tension created by transpiration, the water column of a plant is pulled up passively from roots to great heights.
Factors influencing transpiration
The external factors affecting transpiration are wind, speed, light, humidity, and temperature. The physiological factors affecting transpiration are canopy structure, number and distribution of stomata, water status of plants, and the number of open stomata etc. The advantages of transpiration are as follows:
1. It creates a transpirational pull for absorption and transport in plants.
2. It supplies water for photosynthesis.
3. It cools the leaves and maintains their shape and size.