thigmotropism, the ability of plant organs to change their direction to avoid irritations from one side (such as touching or friction). Haptotropism is characteristic of the tendrils of vines, such as beans and squashes. The growing tendrils possess very high sensitivity to irritation, which is sensed by special epidermal cells. Haptotropism is also observed in the trunks, roots, and leaf stalks of some plant types.