Transpiration is a process that involves loss of water vapour through the stomata of plants. There is a close inter-relationship between transpiration and leaf structure. The rate at which transpiration occurs refers to the amount of water lost by plants over a given time period. Plants regulate the rate of transpiration by opening and closing of stomata. Each stomata has a slit like opening called the stomatal pore, which is surrounded by two special cells called the guard cells. These special cells help to regulate the rate of transpiration by opening and closing the stomata A number of external factors that affect the rate of transpiration, namely: temperature, light intensity, humidity, and wind.
This video explains how to compare the rate of transpiration between the upper and lower surfaces of a leaf.