We have shown that stomata in isolated epidermes show very small responses to light and CO2
. Yet, when an isolated epidermis is placed back in contact with exposed mesophyll of a leaf of the same species or a different species, the stomata respond normally to light and CO2
. Water or silicone oil between the epidermis and the mesophyll closes the stomata immediately. We suggest that there is a signal from the mesophyll to the guard cells that mediates most of the stomatal response to light and CO2.