Some studies have shown that stomata in detached epidennis behave differently, both quantitatively and qualitatively, from those in the intact leaf. Stomata in the intact leaf are very sensitive to environmental factors such as light, CO2 and osmotic stress, but stomata in detached epidermis are less sensitive to these factors than those in the intact leaf. Stomata in the intact leaf open in the light and close in the dark, whereas previously opened stomata in detached epidermis do not close on transfer to darkness and they are dependent on the KCl concentration in the incubation medium. Electrophysiological results also demonstrate different responses of guard cells in the intact leaf and detached epidennis. The electrical potential across the membrane of the guard cell in the intact leaf was hyperpolarised in response to light and CO2. However, there were no changes in the membrane potential difference in response to light and CO2 in detached epidermis. In CCCP-infiltrated leaf discs, the guard cell membrane was conversely depolarised by light, indicating that the guard cell membrane hyperpolarisation was related to the mesophyll cells. These results suggest the participation of the mesophyll in stomatal control. It is possible that stomatal opening in the intact leaf, to a greater or lesser extent. is dependent on an electrical signal or a chemical propagated from the mesophyll.