Measurements of functionally significant xylem variables, such as conduit diameter or vulnerability to embolism, are typically taken from twigs. Ideally, these studies would standardize by distance from the twig tip, given that distance from the terminal end of the conductive stream is the variable that best predicts conduit diameter. To aid efforts to understand the causes of variation in twig conduit diameter, we focus here on the role of leaf length. Longer leaves have wider conduits at the petiole base. It stands to reason that twig tip conduit diameter should closely correlate with petiole base conduit diameter, but this expectation has never been tested. Sampling across 32 orders, 62 families, and 88 species, we show that species with longer leaves have significantly wider conduits at the same distance from the twig tip, as well as lower wood density. Our results underscore the importance of considering both distance from the twig tip and leaf size when comparing functional xylem variables. These findings suggest that selection favoring changes in conduit diameter at the stem tip or base would lead to changes in leaf length and that selection favoring changes in leaf length would involve changes in conduit diameter throughout the shoot xylem.