Always on the bright side

This question has been studied in a cooperation between the Plant Biomechanics Group of the University of Freiburg and the Functional Morphology and Biomechanics Group of the University of Kiel. The upper and lower surfaces of the leaves are covered with small hooks and interlock efficiently with different contact surfaces. That way cleavers can climb on host plants using the leaves as climbing organs.

But owing to a mechanism functioning like a double ratchet cleavers stay on the bright side above the leaves of other plants but slip past them if the leaf is underneath them. The structures underlying this mechanism have also been studied in the project. The leaf hooks of the upper and lower leaf surfaces differ in orientation, distribution, structure and mechanical properties.

In accordance with these differences, friction properties of leaves depend on the direction of the applied force and differ significantly between both leaf surfaces, resulting in the ratchet mechanism.