Contact separation force of the fruit burrs in four plant species adapted to dispersal by mechanical interlocking

Diaspores of many higher plants, adapted to dispersal by attachment to animal fur and feathers, have evolved special structures providing mechanical interlocking or adhesion by glue. In this study, the contact separation force of a single structure, a burr, was used to characterise the attaching ability of the interlocking fruits in four plant species, Agrimonia eupatoria, Circaea lutetiana, Galium aparine, and Geum urbanum. The force that separated a single burr from interlock, the contact separation force of the burr, was measured. The morphological variables of burrs were estimated, and the dependence of the force on the size and the morphology of burrs was examined. The contact separation force of the burr significantly differed among species studied. It ranged from 3.36 to 143.85 mN. The increase of the force is positively correlated with burr length and burr diameter in the basal part. However, in C. lutetiana, the burrs experienced the lowest force, although the values of morphological burr variables were intermediate, and the force applied did not cause the burr damage. In the three species studied, the fruits of which bear many burrs, the force was considerably lower than the fruit weight. To hold the fruit attached to the animal, the interlocking of several burrs (5–21 depending on the species) is required. Total force of all burrs covering the fruit exceeded the fruit weight by 4–20 times. This may increase the attachment force of the fruit, when more than the minimal burr number required are interlocked.