Corner’s rule states that in woody plants, twigs (current-year shoots) with thicker stems support larger leaves. Larger leaf areas require thicker twigs for hydraulic and mechanical support, but a question remains as to why the pattern of thicker twigs resulting in larger leaves emerges, and also as to whether total leaf area should be partitioned into many small leaves or a few large leaves. Corner’s rule implies that larger twig leaf areas should be partitioned into larger sized leaves.
Smith et al. verified Corner’s rule in six co-occurring and functionally similar species, finding that individual increases in leaf size correlate in a strikingly consistent manner with increases to twig leaf area. Supporting their hypothesis with computer simulations, the authors propose that the resulting pattern of leaf formation optimises whole twig photosynthesis, thereby offsetting the cost of the endeavour of producing larger leaves.