Because auxin stimulates shoot elongation, but inhibits root elongation this leads to unequal growth on the two sides of the structure and hence curvature. This is the basis of the Cholodny-Went hypothesis of the mechanism of plant tropic responses, which is not universally accepted although it has been the subject of much research over the past 50 years. Once the rhythm is set up or entrained it will continue for several cycles in total darkness. Mechanical stimulation Some plants such as the senstive plant Mimosa pudica have obvious responses to touch or other mechanical stimulation. Of course plants have no nervous system and it is not clear how they perceive these stimuli.
Electrical signals seem to be involved, just as they are with nerve cells.
Plants under mechanical stress usually produce more ethylene than unstressed plants, and since ethylene inhibits stem elongation it may be involved in the growth response of plants. One of the best places to see plant response to touch or thigmotropism is in the tendrils of climbing plants such as Vitis . In this case the stress is generated by the weight of the plant itself. At the cell level sensing and adjustment to the pressure of the plasma membrane on the cell wall is a necessary part of the cell's water relations. There seem to be pressure-sensing proteins in the cell wall so that water potentials inside and outside the cell and the elasticity of the cell wall can be balanced.