The diagram below represents an experiment to demonstrate a certain phenomenon in a green plant (a leafy shoot fixed by rubber tubing to a glass tube standing in mercury): The diagram below represents an experiment to demonstrate a certain phenomenon in a green plant (a leafy shoot fixed by rubber tubing to a glass tube standing in mercury):
How would the rate of the above process differ if the environment of the plant has: 1. Less humidity 2. High temperature?
Topic: Factors affecting transpiration
Answer
- LESS HUMIDITY — the rate INCREASES. Dry air holds little water vapour, so the difference in concentration between the leaf and the air is large and diffusion out is rapid.
- HIGH TEMPERATURE — the rate INCREASES. Heat supplies more energy for evaporation, and the stomata tend to open wider.
Both conditions INCREASE transpiration, and for each you should state the direction AND the reason.
The four external factors, and note that HUMIDITY works in the OPPOSITE direction to the other three: TEMPERATURE up — transpiration UP LIGHT up — transpiration UP (stomata open) WIND up — transpiration UP (vapour carried away) HUMIDITY up — transpiration DOWN
THE UNDERLYING PRINCIPLE: evaporation depends on the difference in water-vapour concentration between the inside of the leaf and the air outside. Anything that WIDENS that gap speeds transpiration; anything that narrows it slows transpiration.