Vessel length measurements
Maximum vessel length of stems and leaf petioles was determined by applying the air injection method (Greenidge, 1952). After connecting the basipetal part of a stem or petiole to a syringe, a ca. 150 kPa pressure was applied, while the acropetal part was kept under water. Successive cuts at the proximal stem/petiole end were made under water with a razor blade at intervals of 10 mm for stems and 2 mm for leaves until the first continuous stream of air bubbles could be seen emerging from the cut end. The corresponding length was then measured and recorded as the maximum vessel length of a stem or leaf petiole (MVLstem and MVLpetiole, respectively). At least six stems or leaves were taken for these measurements (Figure S1).
We measured the maximum vessel length at the stem-leaf transition for leaves attached to 0.5 cm long stem segments. This approach was applied to six samples per species based on the air injection method. The syringe was connected to the short stem sample, and the leaf was shortened until bubbles could be seen emerging from the cut end.
The vessel length distribution of leaf xylem was measured with a Pneumatron device (Pereira et al. , 2020b). Instead of injecting air (Cohen et al. , 2003; Wang et al. , 2014; Pan et al. , 2015), the amount of gas that could be sucked up via cut-open conduits allowed us to measure the air conductivity of open vessels while shortening leaf petioles. We then plotted the air conductivity of the cut-open vessels against the petiole length. The average vessel length was obtained by fitting the vessel length equation from Sperryet al. (2005) to our data.
We defined a segmentation index as the maximum vessel length at the petiole end divided by the petiole length. This index indicated to what extent the longest vessels from the petiole end run into the leaf blade. Values < 1 indicated that vessels ended before the leaf blade started, while values > 1 suggested that at least some vessels starting at the petiole end run directly into the midrib of the leaf blade.