Xylem diameter of petioles and peduncles
The xylem lumen diameter of three petioles and three peduncles from each of three additional plants was examined to test for any association between organ xylem vulnerability to cavitation and vessel size. Transverse sections of peduncles and petioles were made close to the base where they joined the main stem using a sliding microtome (Leica Microsystems, North Ryde, NSW, Australia) and a BFS‐3MP Freezing Stage (Physitemp Instruments, Clifton, NJ, USA) at 30 µm thickness. Cross sections were stained with toluidine blue (5%) and mounted on glass slides with phenol glycerine jelly. Slides were photographed at 20x magnification using a Nikon Digital Sight DS-Fi2 (Tokyo, Japan) mounted on a compound microscope (DM1000; Leica Microsystems, Wetzlar, Germany). Vessel diameter was measured using ImageJ. Four representative fields of view (20x magnification) of entire xylem bundles were captured for each transverse section.  Using IMAGEJ, a binary image was produced for each section, and vessel lumens were selected using the threshold function. An ellipse was automatically fitted to each xylem vessel lumen (fig. 1). The lumen area of each vessel was automatically measured using the ‘analyse particles’ function to calculate an average cross-section diameter for each vessel. Given that the flow rate of water through vascular tissue is directly proportional to the xylem diameter to the 4th power (Hagen–Poiseuille law), only the largest of xylem vessels contribute significantly to the water transport capacity of the vascular system (Petit, Anfodillo, & De Zan, 2009; Melvin T. Tyree & Ewers, 1991). Thus, for each section, xylem diameters were ranked and the mean diameter of the largest 5% of vessels measured. This allowed us to compare the theoretical water supply capacity and resilience to xylem collapse in vegetative and reproductive support structures.