Xylem conduit morphology is shaped by the challenges of minimizing hydraulic resistance and preventing conduit wall collapse during vertical sap transport. While hydraulic theories predict that conduits widen from tip to base to minimize resistance, theory has not addressed how collapse prevention influences vertical variation in conduit morphology. Additionally, scaling relationships in roots remain largely unexplored. Here, we evaluate existing theories for conduit diameter scaling and synthesize new theory for vertical variation in thickness-to-span ratios. We test these theories using a novel bootstrapping approach to minimize sampling biases and analyze a data set of nearly 600 000 xylem conduits spanning above- and belowground organs from five conifer species. As predicted, conduits widened with distance from the leaf tip, with scaling exponents closely aligning with theoretical predictions. Conduits also widened from fine roots to coarse roots, mirroring aboveground patterns. Thickness-to-span ratios increased from base to tip and consistently exceeded the predicted critical collapse limit. These findings reveal how the physics of sap transport shape xylem morphology to balance hydraulic efficiency and structural stability. By combining novel theory, robust statistical methods, and comprehensive data, this study refines scaling predictions and advances understanding of mechanisms shaping xylem anatomy across plant organs.