Society relies on mathematical descriptions of climate change, weather forecasting, crop performance, and other processes in which the control of carbon uptake and water loss by plants forms a basic element. Scientists also use the same element in ascribing sources and sinks of carbon dioxide (CO₂), describing vegetation, hydrological and ecological processes. A key physiological assumption in this element is now in doubt and we will test it rigorously and if necessary provide a robust alternative. We will do this by developing a novel 'window' on intact leaf functioning that will reveal the concentration of water vapour and other gases inside leaves.