Organisms make decisions when they perceive cues of varying intensities. In case of climbing plants, the diameter of supports in contact (tree or stem) is an important cue for their growth as plants that coil around a support with large diameter are unable to maintain tensional forces required for continued attachment to the support. The negative association between the diameter and the climbing success has been reported since Darwin published his study on climbing plants. However, it is not known if a climbing plant makes a decision to avoid a support with larger diameter. Here, we tested this possibility by observing the coiling response of tendrils of Cayratia japonica to supports with different diameters. The coiling success of the tendrils was affected by the diameter of the support and the tendril lengths. We propose a decision tree to describe the different phases of the coiling response and demonstrated that the tendrils change their coiling shape depending on the support diameter and the tendril length. To understand the behavioural rules regulating the phase pattern, we constructed a simple model with two assumptions on the tendril movement, (1) when the tendrils receive a contact stimulus, they begin to coil from around the contact point and (2) there is a minimum coiling angle at which the tendrils coil up, once the tendril starts coiling. Image analysis and 3D motion tracking technique revealed that the movement of the tendrils were consistent with the two assumptions of the model. The results suggested that the tendrils flexibly changed the coiling shapes depending on the support diameter and simple behavioural rules could regulate this diameter-dependent response.
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