Branch or leave angle climbers are climbing plants which attach to their neighboring plants by wide-angled stiff branches or leave petioles interlocking with the branches of the neighboring plants. Many of such branch angle climbers are characterized by "sticky" or "adhesive" leaves which increase the frictional contact with the neighboring plants. Here we analyse the structure and function of the leaves of Galium aparine, a typical leave angle climber attaching to neighboring plants by wide-angled "adhesive" leaves. The adhesive properties of the leaves are due to the existence of hooks on the abaxial and adaxial leave surface.
In order to analyse the structure and function of the adhesive leaves of G. aparine we have tested the attachment properties of the leaves on different referential surfaces, performed tension tests on single hooks, and studied the orientation, distribution, structure and anatomy of the leave hooks. The structure and properties of the leave hooks differ conspicuously between hooks on the abaxial and adaxial leave surface. On the abaxial leave surface hooks are curved towards the leave basis, are situated exclusively on the midrib and on the borders of the leave, continuously lignify and are hollow. On the adaxial leave surface hooks are curved towards the leave tip, distributed evenly over the whole leave surface, lignify predominantly at the apex and are narrower than the hooks of the abaxial leave surface. Furthermore, tensions test on single hooks showed that hooks on the abaxial leave side display a significantly higher mechanical stability than hooks on the adaxial side. In accordance with these differences in orientation, distribution, structure and mechanical properties of the hooks, attachment properties of the G. aparine leaves 1) depend on the direction of the applied force and 2) differ substantially between abaxial and adaxial leave surfaces. When leaves are dragged against a referential surface in the direction opposite to the curvature of the hooks the generated frictional forces are minimal or not measurable at all. When leaves are dragged against a referential surface in the direction of the curvature of the hooks, frictional forces are significantly higher.
Due to the dependence on the force direction and the differences between abaxial and adaxial leave side, the leaves of G. aparine interlock with their abaxial surfaces on the leaves of neighboring plants. On the contrary, the adaxial surfaces of the leaves slip of the leaves of neighboring plants. This entails that the leaves of G. aparine are always positioned upon the leaves of the neighboring plants and do not get attached underneath them. Therefore, the leaves of G. aparine can function as attachment organs, and simultaneously orientate themselves properly for their photosynthetic function. In a current study, the structure and function of adhesive leaves of several tropical branch angle climbers are analysed and compared to the results obtained in G. aparine.