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Summary 1. Stomata have been found to respond to small changes in carbon dioxide concentration within the leaf, and movements due to illumination, temperature, leaf‐water content, or metabolic inhibitors can be explained, at least in part, by their affecting the internal carbon dioxide concentration. This is confirmed by the fact that effects of such factors can be reversed experimentally by flushing the leaf with air of an appropriate carbon dioxide content. We conclude that changes in carbon dioxide concentration set in motion reactions affecting guard cell turgor relations (see 8 below). 2. The photosynthetic production of carbohydrates, or of intermediates in their synthesis such as glycollic acid, does not seem to have any major effect on guard cell turgor, which can readily change in the absence of photosynthesis in darkness in response to different carbon dioxide concentrations. Factors which affect photo‐synthetic production ordinarily affect the internal carbon dioxide concentration and it is the latter which is more directly involved in changing guard cell turgor relations. 3. In addition to the effects of environmental factors on carbon dioxide concentration, light (blue), temperature and leaf water content may also affect guard cells independently of carbon dioxide. Whether such effects are due to hydrolysis of starch or another polysaccharide, or to changes in permeability or quite another mechanism, is not known. 4. As well as being effected by the environment, stomatal movements are under the control of endogenous rhythms in light and darkness. Rhythms can produce opening in darkness and partial closure in light, and hence can modify or overrule the response to external factors. However, the phase of the rhythm does come under the control of the environment through a low‐intensity light reaction sensitive to red and far‐red wavelengths. This light reaction is quite distinct from those directly concerned in the production of stomatal opening. 5. Stomatal behaviour in cacti and other succulents is at first sight practically the opposite of that in other plants, in that they open at night and close during the day. We suggest that this type of behaviour may, however, be a relatively simple development from the normal pattern. 6. Stimuli affecting the stomata can be transmitted within a leaf, or from one part of a plant to another. The mechanism of the transmission is not known, but it could be brought about by a chemical substance which is translocated. 7. There is evidence that the processes of stomatal opening and closing are different in nature, and that one is not simply a reversal of the other. It is probable that an active (oxygen‐requiring) mechanism in involved in stomatal opening. Stomata come under the control of so many factors, both external and internal, that several processes are likely to contribute to the turgor changes which bring about their movements. 8. The mechanism of carbon dioxide control over stomata is not known.

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