Recent methodological advances toward measuring sap flow on small- diameter stems allow for near continuous monitoring of leaf-level water flux in response to naturally varying environmental conditions. Elucidating the response functions of sap flow to environmental variables is necessary for accurately understanding and modeling the processes underlying leaf transpiration. In main stems, hysteresis often occurs in the response of sap velocity to vapor pressure deficit (VPD). Hysteresis occurs when, for a given VPD that occurs in both morning and evening, morning sap velocity is higher than it is in the evening, producing a clockwise rotation in the hysteresis curve. Here we ask whether the relationship between sap velocity and VPD for leaf petioles is similar to that for main stems. We measured sap velocities using the heat ratio method on leaf petioles of the tropical tree Cordia alliodora (Boraginaceae) and the tropical liana Clitoria javitensis (Fabaceae). In most cases, we found clockwise patterns of hysteresis in the sap velocity-VPD relationship similar to those previously published for main stems. However, on some days, we observed a figure-8 pattern of hysteresis in the relationship of sap velocity to VPD. Furthermore, we examined what may cause variation in the amount of hysteresis on a given day. We found a significant positive relationship between the magnitude of hysteresis and both the maximum daily VPD and the integrated daily VPD. These results suggest that the type and magnitude of hysteresis may result from an interplay between environmental variables and plant hydraulic architecture.