The movement of part of a plant in response to external stimulus of no particular direction is?

Analyze the question: It asks for the term for a plant part's movement in response to an external stimulus where the direction of the stimulus does not decide the direction of the movement.

Explain the concept: Plant responses to stimuli fall into two main groups:

• Tropism is a growth movement where the direction of the stimulus determines the direction of the response. Examples are phototropism (a shoot bending toward light) and geotropism (a root growing downward).
• Nastic movement (nasty) is a response to a stimulus where the direction is not important. The direction of movement is fixed by the structure of the plant organ, not by where the stimulus comes from. Examples are the folding of Mimosa pudica (the "touch-me-not" plant) leaflets when touched, and the opening and closing of flowers in response to light or temperature.
• Taxism (taxis) is the movement of a whole free-living organism, such as Chlamydomonas or sperm cells, toward or away from a stimulus. It is not a movement of a part of a plant.
• Haptotropic movement is a tropism in response to touch or contact, such as a tendril coiling around a support. It is directional, because the tendril coils around the object that touched it.

Solution:

1. "Part of a plant" rules out taxism, which involves the whole organism moving.
2. "No particular direction" rules out tropism and haptotropism, which are both directional.
3. That leaves nastic movement, where the stimulus acts without determining the direction of the response.

Answer: The correct option is D. nastic movement.

Exam tip: Remember "Tropism = Towards/away from the stimulus" and "Nastic = Not directional". The words no particular direction in a question almost always point to nastic movement. Also remember Mimosa pudica as the classic example.