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).