Six races of Sesleria caerulea from widely separated parts of the geographical range of the species exhibited different rates of photosynthesis and transpiration when grown under standard conditions. These can be traced to various attributes of the stomatal apparatus. The stomata differ in frequency, size and the depth to which they are sunk below the epidermal surface in the different races, but quantitative consideration of these parameters leads to the conclusion that the apertures to which the stomata can open, an attribute which correlates with the depth to which they are sunk, determines the maximum rates of photosynthesis and transpiration. Evidence is presented to suggest that, although this complex of stomatal modifications may be interpreted as representing xerophytic adaptations, they may have evolved not so much for conservation of water per se, but to conserve the energy balance of the leaves, thereby reducing evaporative heat loss and so maintaining leaf temperature. A leaf temperature of 1–2°C above ambient may increase the rate of photosynthesis by 10–20% in the range from 10–20°C.