Petiole vessel length measurements
L.
tulipifera and Q. petraea had the longest petiole vessels of the
six species studied, with an average length of 8.19 ± 1.47 and 8.02 ±
1.33 cm (mean ± standard deviation), 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 of B. 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).