Corner’s rules describe a global spectrum from large-leaved plants with thick, sparingly branched twigs with low-density stem tissues and thick piths to plants with thin, highly branched stems with high-density stem tissues and thin piths. To examine why, Olson et al. sampled 55 species from a Mexican tropical dry forest and use structural equation modelling to correlate leave size and spacing with stem anatomy.
Large leaves must be spaced farther apart, avoiding self-shading. Farther spacing means that large-leaved plants make longer stems with less material, so wood in stems of large-leaved plants is more flexible. For a given amount of material tubes are stiffer than solid cylinders, as in bike frames, explaining why species with larger leaves, with their flexible wood, make thicker twigs with wide piths.
Olson, M. E., Rosell, J. A., Zamora Muñoz, S., & Castorena, M. (2018). Carbon limitation, stem growth rate and the biomechanical cause of Corner’s rules. Annals of Botany, 122(4), 583–592. https://doi.org/10.1093/aob/mcy089