What is the stimulus involved in the process of thigmotropism?

How does the plant detect the touch,and how do the leaves move in response?

$(a)$ How are the movement of leaves of touch-me-not and the growth of a stem towards light different?
$(b)$ What is chemotropism? Give one example.
$(c)$ What is tropism?

Tendrils encircle or coil around the object in contact with it. Elaborate.

Which plant organ shows negative phototropism?

The substance that triggers the fall of mature leaves and fruits from plants is due to