Transpiration is the loss of water through the leaf. There are tiny openings called stomates and these are surrounded by guard cells. In dry conditions, the guard cells generally cause the stomates to close, thereby keeping water loss to a minimum. However, if the guard cells swell and the stomates are open, transpiration will occur and water vapor will exit the leaf. This can be seen by examining the leaf under the microscope to see stomates in the open position and also, by further examining individual cells, the vacuoles that store water in the plant will shrink due to water loss. These vacuoles work like storage compartments and when the leaf cells have a lot of water, they take up a great deal of room within the confines of the cell. When transpiration occurs and water is lost, the vacuoles are much smaller in volume.