(thigmotropism) A *tropism
in response to an external contact stimulus. The movements are most clearly exhibited by the tendrils of certain species. In passion flowers (Passiflora), the tendril is coiled spirally with the lower side facing outwards. The tendril later straightens and the sensitive tip undergoes *circumnutation
. If it touches a solid body, it bends towards the stimulated side, a process occupying perhaps several minutes. The bending results in fresh parts of the tendril making contact with the stimulus and so the process is continuous, the tendril gradually encircling the support.