Growth or movement of a cell or an organism in response to touch
In general, tropism is an orienting response of an organism to a stimulus. It often involves the growth rather than the movement of an organism. The response exhibited by the organism to the stimulus is mostly involuntary, which means it cannot go either towards or away from the stimulus source as in taxis. In tropism, the response of the organism is often by its growth rather than by its movement. It may grow either towards or away from the stimulus.
Thigmotropism is an example of tropism and it may be positive or negative. A positive thigmotropism is a response towards the touch stimulus whereas a negative thigmotropism is a response away from the touch stimulus.
Examples of positive thigmotropism are the growth of ivy on walls upon contact to walls and the coiling of tendrils or twiners upon contact to objects for support. An example of negative thigmotropism is the growth of roots underneath the soil. When an elongating root comes in contact with an object, e.g. rock, it grows away from the object.
The drooping of leaflets of Mimosa pudica when touched is not considered thigmotropism but a nastic movement. It is because the response of this plant is not influenced by the direction of stimulus, which is a characteristic of thigmotrophic movements.
Word origin: Greek thigma (touch) + tropism