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):
Will the level of mercury in the glass tubing rise or fall? Which conducting tissue of the plant does the glass-tubing represent?
Topic: Potometer — mercury level and xylem
Answer
The mercury level will RISE. The glass tubing represents the XYLEM.
As the shoot transpires, water is lost from the leaves. This creates a SUCTION — the transpiration pull — which draws water up the glass tube, and the mercury below it rises with the column.
The glass tubing represents the XYLEM because it is the channel along which water is drawn upward, exactly as the xylem carries water from root to leaf.
THE PRINCIPLE the experiment demonstrates: transpiration at the top is what lifts water at the bottom. The rise of the mercury measures the rate of water uptake, which is used as a measure of the rate of transpiration.