Petiole vessel length measurements
L. tulipifera and Q. petraea had the longest petiole vessels of the six species studied, with 8.19 ± 1.47 and 8.02 ± 1.33 cm, respectively. Despite having the longest petioles and the largest leaf surface area, vessels of L. tulipifera were always shorter than the petiole length, indicating that vessels at the base of the petiole ended well before the base of the lamina. Cut-open vessels in the petiole ofB. pendula also ended before the base of the lamina. Petiole vessels of Q. petraea , however, were found to run from the petiole base up to the middle of the midrib. A similar observation was found for cut-open vessels in F. sylvatica and P. avium , which reached to half the length of the midrib. C. betulus had petiole xylem with vessels that were only slightly longer than the petiole (Table 1).
Q. petraea had the longest average vessel length in petiole xylem, with a value of 5.1 ± 0.9 cm, which was followed by L. tulipifera (3.52 ± 1.17 cm). The mean vessel length in petioles ofB. pendula , F. sylvatica , and C.betulus was shorter than 1 cm. Since the petiole of P. avium had a pronounced notch, we were unable to obtain accurate vessel length measurements for this species with the Pneumatron, even when using glue or parafilm to avoid any leakage (Table 1).