Why the leaves of cold temperate deciduous and moisture-loving angiosperms are so often toothed has long puzzled biologists because the functional consequences of teeth remain poorly understood. Here we provide functional and structural evidence that marginal leaf teeth of Chloranthus japonicus, an understory herb, enable the release of guttation sap during root pressure. When guttation from teeth hydathodes was experimentally blocked, we found that the leaf intercellular airspaces became flooded. Measurements of chlorophyll a fluorescence revealed that internal flooding resulted in an inhibition of photosynthesis, most likely through the formation of a film of water within the leaf that reduced CO2 diffusion. Comparing a developmental series of leaves with and without teeth experimentally covered with wax, we found that teeth did not affect overall leaf stomatal conductance and CO2 uptake. However, maximal and effective light-saturation PSII quantum yields of teeth were found to be lower or equal to the surrounding lamina throughout leaf ontogeny. Collectively, our results suggest hydathodes and their development on teeth apices enable the avoidance of mesophyll flooding by root pressure. We discuss how these new findings bear on the potential physiological interpretations of models that apply leaf marginal traits to infer ancient climates.