Rate of Transpiration

Various external and internal factors influence the rate at which transpiration occurs. This rate of transpiration can be measured with a potometer.

In a potometer, the stem is cut and attached to a capillary tube full of water. The rate of water uptake is measured by adding an air bubble into the capillary tube and then timing how long it takes the bubble to move a certain distance. This of course is assuming that the rate of transpiration is equal to the rate of uptake of water.
The internal factors are mostly connected with the stomata - their numbers, structural features, distribution and how open they are. External features need to be looked at with more detail:

Temperature - A high external temperature will create evaporation from the mesophyll cells of the leaf.

Humidity - This is how saturated the surrounding air is with water vapour. It will determine the humidity difference between the inside and outside of the leaf(called the saturation deficit). Normally the humidity just inside the stomata is very high as it is full with water vapour. It will move out fastest if there is a steep concentration gradient, ie if the surrounding air has a low humidity. High external humidity will cause a slow transpiration rate.

Wind - If there is little air movment, the water vapour leaving the stomata will tend to build up around the stomata. This will create a high humidity right outside the stomata, and so transpiration will be less. So the more windy it is, the faster transpiration will occur.

Atomospheric Pressure - The lower the atmospheric pressure, the greater the rate of evaporation.

Light - When the light is increased, the stomata open wide, allowing a lot of transpiration.

Water Supply - For transpiration to occur, the walls of the mesophyll cells need to be wet. So the plant needs a decent water supply. If it can't get enough water, the stomata close to conserve water - reducing the rate of transpiration.