3.2 Vascular geometry in the leaves of Austrobaileya
scandens
Mature leaves of A. scandens (Figure 2a) are coriaceous, with a
short petiole and an entire lamina. The vasculature of the petiole
displayed as adaxial xylem tissue composed mainly of squared tracheids,
and abaxial phloem composed of sieve tubes (identified by the presence
of callose), rays, and parenchymatous tissue (Figure 2b). While fiber
caps were lacking in the petiole, a massive protective perivascular
fiber cap proliferated in the midrib (Figure 2c-e). The cross-sectional
areas of both the xylem and the phloem were greatest in the petiole and
narrowed linearly toward the tip of the major vein [xylem:
y=-0.05x+0.20(mm2), phloem:
y=-0.04x+0.15(mm2)]. Strikingly, conduit diameter of
both phloem and xylem were invariant along the midrib, but the number of
conduits decreased by a factor of 2.6 from the petiole to the midrib
tip.
Xylem and phloem conduits varied in size across vein hierarchies (Figure
3a), gradually decreasing in size from major to minor veins, except in
the petiole, where tracheids were shorter and narrower than those of the
midrib (Figure 3b). Although sieve tube elements were also shorter in
the petiole, they were larger in diameter with respect to midrib (Fig.
3C). Sieve plate pore radii varied from 0.12 µm (±0.007 SE) in the
petiole to less than 0.08 µm (±0.0006 SE) in the minor veins (Figure
S1). Functionally, foliar phloem architecture correlated with a low bulk
velocity of the mobile dye tracer esculin hydrate in vivo (Fig.
4; Video S1), revealing an average rate of 11 µm s-1in the second order veins (n=2).