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).