$(a)$ 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?

$(N/A)$ The movement of leaves of the touch-me-not plant $(Mimosa \, pudica)$ is a nastic movement, which is fast and non-directional. In contrast, the growth of a stem towards light (phototropism) is a tropic movement, which is slow and directional.
$(b)$ Chemotropism: It is the growth or movement of a plant part in response to a chemical stimulus.
Example: The growth of the pollen tube towards the ovule during fertilization.
$(c)$ Tropism: It is a directional growth movement of a plant part in response to an external stimulus.