Manipulated (IV): humidity (which can be changed by sealing the shoot inside a plastic bag. As water vapor is lost, the humidity in the bag increases. Humidity can be measured with a hygrometer.) dependent (responding) variable: water uptake (measured using a potometer)
The controlled variables are: temperature, air movement and light.
Humidity: if the humidity of air increases, the rate of transpiration decreases. Increasing air humidity decreases the diffusion gradient between leaf air spaces and the air outside.
Temperature: if the temperature increases, the rate of transpiration increases. A higher temperature increases the evaporation of water from mesophyll cells, lowers the relative humidity of air, and increases the rate of diffusion.
Air movement: if the air movement increases, the rate of transpiration increases. Water vapor that exits the leaf is rapidly moved away, maintaining a high diffusion gradient.
Light: in light, stomata open, when dark they close.