Data for pith width, leaf size, and twig thickness of trees and shrubs

Premise of the study: As an extension of E. J. H. Corner’s Rule that twig diameter increases with leaf size, we hypothesized that pith width also increases with leaf size. The benefit to the plant from the proposed relationship is that pith is a low-cost tissue that reduces the metabolic cost of large diameter twig production. Methods: Leaf sizes and cross-sectional areas of bark, xylem, and pith of 81 species of trees and shrubs growing in Gainesville, Florida were measured and compared with standardized major axis regressions of pairwise species trait values and phylogenetically independent contrasts. Key results: Pith area increases with leaf size with or without accounting for phylogenetic relationships. In agreement with Corner’s rule, overall twig diameter as well as bark and wood thickness also increase with leaf size. Thicker twigs showed more variation in relative pith, wood, and bark cross-sectional areas compared to thinner twigs. Conclusions: Investments in pith, a tissue of low density found in the centers of twigs, provides a low cost way to increase twig circumference and thereby space for attachment of large leaves while increasing the overall second moment of area of twigs (I), which increases their ability to biomechanically support large leaves.

研究前提：作为E. J. H. 科纳法则（E. J. H. Corner’s Rule）——即小枝直径随叶片尺寸增大而增加——的延伸，我们提出假设：髓宽同样随叶片尺寸增大而增加。该关联对植物的益处在于，髓是一种低成本组织，可降低大直径小枝构建的代谢成本。
研究方法：我们测量了生长于美国佛罗里达州盖恩斯维尔地区的81种乔木与灌木的叶片尺寸，以及树皮、木质部和髓的横截面积，并通过标准化主轴回归（standardized major axis regressions）对物种成对性状值与系统发育独立对比（phylogenetically independent contrasts）进行了比较分析。
主要结果：无论是否考虑系统发育关系，髓面积均随叶片尺寸增大而增加。与科纳法则一致，总体小枝直径以及树皮和木质部厚度同样随叶片尺寸增大而增加。相较于细弱小枝，粗壮小枝的相对髓、木质部和树皮横截面积的变异程度更高。
研究结论：作为分布于小枝中心的低密度组织，对髓的投入是一种低成本途径，可增加小枝周长，从而为大型叶片附着提供空间，同时提升小枝的整体截面二次矩（second moment of area），进而增强其生物力学支撑大型叶片的能力。