Stomatal apertures were measured at midday on sunny days on leaves of a number of different species growing at day temperature ranging from 15 to 36 °C. In most of the species the apertures increased with increasing air temperatures up to 36 °C. Two cool climate species, pea and broad bean, had the widest apertures at about 27 to 30 °C, and the apertures decreased slightly at higher temperatures. Stomata do not appear to close at midday in leaves of most species at air temperatures of 36 °C or lower provided that the leaves are not under water stress.