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Leaves of hornbeam (Carpinus betulus) and beech (Fagus sylvaticus) were modelled to a first approximation as plane surfaces, with straight parallel folds, using numerical methods. In both species the lateral veins, when the leaves are outstretched, are angled at 30 to 50 degrees from the centre vein. A higher angle allows the leaf to be folded more compactly within the bud, but it takes longer to expand. This may allow the plant to optimize the timing of leaf deployment with ecological and physiological conditions.
KOBAYASHI et al. (Thu,) studied this question.