Discussion
Woody seedlings are a convenient life stage to acquire water conductance parameters at the whole-plant level because of their size advantage, even though patterns of seedlings may could not completely reflect the water relation in adult woody plants. Nevertheless, how woody seedlings regulate water relations, in terms of the xylem-stomatal covariation, is important for their survival and growth into adulthood. Based on an anatomical analysis across ontogenetically comparable seedlings of 53 diverse woody species (Table S1), we have presented here key new findings on xylem-stomata coordination from a previously neglected aspect: we started from the water balance between the liquid (water delivery) and vapor (water loss) phase at the whole-plant level by scaling plant total stomatal area to stem xylem (conductance) cross-sectional area (Fig. 2; Table 1). This area-scaling pattern was driven by the covariation of stomata numbers and minor vessel numbers per plant (Fig. 5a; Table 1). We then zoomed in on the water exchange in individual leaves by showing the coordination of leaf total stomatal area and midvein xylem area (Fig. 3; Table 1). We also found that plant size (or leaf size) scales with stomatal (or minor vessel) number, while it does not scale with individual stomatal (or minor vessel) area (Fig. 4; Table 1), which has an important implication for our understanding of the design of xylem structure and stomatal distribution.