Leaf temperatures associated with torn and untorn leaves of Musaceae were taken in both dry and wet seasons at Barro Colorado Island, Canal Zone. Transpiration rates and leaf resistance to water—vapor diffusion were determined. Energy—budget analysis is used to describe the relationship of leaf dimension and leaf resistance to thermal survival and water usage. Gas—exchange theory is applied to predict the photosynthetic implications of leaf tearing. The analyses showed that leaves less than 10 cm wide are not subject to critical heat stress, have lower water loss, and higher ratios of photosynthesis to water expended than do leaves of widths greater than 10 cm. In wet season, leaves have lower resistance to the diffusion of water vapor and accordingly are less subject to excessive heating.