Dr Cowan has made outstanding theoretical and experimental contributions to knowledge of gas exchange in higher plants as influenced by external physical, and internal physiological, processes. His application of control theory has led to a profound understanding of the system in which uptake of carbon dioxide and loss of water vapour are interrelated and regulated through the action of stomata in plant leaves. His hypothesis of optimal control, which represents the way in which the regulation of carbon gain and water loss is modified to withstand drought, is a major step in linking physiological responses to ecological distribution and productivity and has important implications to the adaptation of plants to dry conditions.