A student sets up a potometer to estimate the rate of transpiration of a leafy shoot, as shown in the diagram.
What is the main function of the water-filled reservoir, labelled X?
To maintain a constant level of humidity around the leaves.
To allow the air bubble to be reset to the start of the scale for repeat trials.
To measure the head of water pressure driving transpiration.
To prevent any water from evaporating from the cut end of the stem.