CBSE Class 11-science Answered

• The loss of water in the form of vapour from the aerial parts of the plant is called transpiration.
• Factors such as temperature, light, humidity, wind speed affect the rate of transpiration.

Mechanism of Opening and Closing of Stomata

• The surfaces of spongy mesophyll cells in the leaf are exposed to intercellular spaces.
• These cells give out a thin film of water. Water from this film evaporates.
• Water vapour formed saturates the air in the intercellular spaces, diffuses into the connecting intercellular spaces and reaches the sub-stomatal space. Finally, it escapes in the atmosphere through the air.

• Water absorbed by the roots rises through the stem and reaches the tissues of leaves.
• The cell sap in each cell exerts a turgor pressure outward on the cell wall. This pressure forces some water out of the cell wall into the air space between the cells, i.e. intercellular space.
• Here, the water evaporates and the water vapour from the intercellular spaces diffuses into the sub-stomatal space from where it finally goes out by diffusion through the stomata.
• There are more stomata on the undersurface of a dicot leaf; therefore, more transpiration occurs from the undersurface.