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.