When the relative humidity in a plant's environment rises, its rate of transpiration lowers, and a decrease in humidity causes the transpiration rate to rise. Air that is humid does not accept water vapor easily, and drier air makes it easier for a plant to release water by evaporation through the stomata on its external surfaces. Because drier air can account for drier soil conditions, the water uptake portion of the transpiration process may also increase as the plant attempts to bring in more water through its root system.
Transpiration refers to the movement of water through a plant and includes the uptake of water through its roots and the release of water through the stomata. The stomatal pores open and close in response to environmental changes and conditions within the plant, and they play an important role in regulating transpiration. The stomata also enable the release of carbon dioxide, which along with water, is a byproduct of photosynthesis.
Humidity is not the only environmental factor that can affect a plant's transpiration. Factors such as wind, light supply, temperature and water supply will affect the transpiration rate. Physical characteristics of the plant will also affect the process. These can include the number and the size of the leaves on the plant, the number of stomata and the thickness of the cuticle, which is a waxy and water-repelling coating that is found on the external surfaces of many plants.