studies and species (Roden & Ehleringer 1999; Cernusak et al. 2003; Loucos et al. 2015; Song et al. 2015; Holloway-Phillips et al. 2016). Adherence to the Péclet effect model appears to depend on the relative importance of enriched vein water to bulk leaf water and the interaction between xylem and mesophyll water pools (Song et al. 2015; Holloway-Phillips et al. 2016).
Leaf anatomy was shown to play a distinct role in Δ18OLW, where parallel venation in grasses increased the magnitude of enrichment from the base of a grass leaf blade to the tip (Helliker & Ehleringer, 2000). Even though Δ18Oeremains the same over the length of the leaf blade, δ18OSW (xylem water) becomes progressively more enriched up the blade, supplying increasingly more enriched water to the sites of evaporation and creating a gradient in δ18OLW. Vein density increased the influence of the enriched mesophyll water on the xylem, increasing the magnitude of δ18OLW along the blade. The string-of-lakes model described this progressive enrichment up the leaf blade as a series of interconnected pools where the source of one pool is the preceding pool (Gat & Bowser 1991; Helliker & Ehleringer 2000). The Farquhar-Gan model was created, so that separate Péclet effects were associated with xylem and mesophyll (lamina) water pools (Eqn. 10; Gan et al. 2003; Holloway-Phillips et al. 2016).