Fig. 1.1 shows apparatus that can be used to measure the loss of water vapour from leaves. [Figure 1.1] The water vapour given out by the area of leaf under the cup increases the pressure inside the tubing, causing the water level in the graduated tube to go down. Some students used the apparatus in Fig. 1.1 to test the hypothesis: The loss of water vapour from the lower surface of the leaves of a plant is greater per unit time than the loss of water vapour from the upper surface of the leaves of the same plant.