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S ummary Although several workers have published accounts of stomatal studies with detached epidermis, there has not been general agreement as to whether the stomata continue to respond to environmental factors as they do in the intact plant. This paper reports a critical examination of the value of the technique, experiments having been carried out on material from Vida faba and two species of Commelina . The nature of the incubating medium was found to exert a considerable influence on the behaviour of the stomata. In Commelina , the presence of monovalent cations (K + , Na + ) stimulated opening, but divalent cations (Ca 2+ , Mg 2+ ) had an antagonistic effect suppressing opening or even preventing it altogether. In Vicia faba cations exerted less noticeable effects. pH changes affected aperture, but in Commelina the changes were of much smaller magnitude than those induced by the different cations. Factorial experiments were performed to determine the effect of detaching epidermis on the stomatal responses to environmental factors. In Vicia faba the light effect was very apparent on attached epidermis, but on detached epidermis the effect was largely obscured by stomatal opening that occurred in darkness; effects of CO 2 concentration were detectable on epidermal strips, particularly in darkness, but were of smaller magnitude than those on attached epidermis. On the other hand, in Commelina , effects of both light and CO 2 were of considerable magnitude when strips were immersed in media containing the cations that stimulated opening. The theory that the opening stimulated by monovalent cations is the outcome of their being actively taken up into the guard cells is discussed, and it is concluded that while this might apply to epidermal strips in media, there is inadequate evidence that such a mechanism is responsible for opening in the intact plant; there was no stimulation of opening on attached epidermis when pieces of leaf were immersed in media containing monovalent cations. Attempts are made to explain the difference in behaviour between Vicia faba and Commelina on the basis of the course of ion uptake into the cells of the epidermis; evidence of a possible difference in uptake was obtained from studies of the penetration by neutral red. It is concluded that epidermal strips from both species of Commelina are suitable for studies of stomatal responses to Hght and CO 2 so long as they are incubated in a suitable medium. Epidermal strips from V. faba are unsuitable for studies of the light responses, and less satisfactory than Commelina for the responses to CO 2 .