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Describe the external factors affecting transpiration.
Answer
Extermal factors affecting transpiration are :

1. Temperature : Rate of transpiration is directly proportional to the temperature. Transpiration rate increases with increase in temperature. 

2. Light : More light closes the stomata.

3. Relative humidity : Rate of transpiration is inversely proportional to the relative humidity.

4. Velocity of the wind : The rate of transpiration is directly proportional to the velocity of wind. However, the wind of 40-50 km/hr brings closing of stomata and thus decreases the rate of transpiration.

5. Atmospheric pressure : Rate of transpiration is inversely proportional to the atmospheric pressure.