Sensitive Plant (Mimosa pudica):
* Mechanism: The movement is caused by changes in turgor pressure within specialized cells at the base of the leaflets called pulvini. When the plant is touched or stimulated, these cells rapidly lose water, causing the leaflets to fold inward and the petiole to droop.
* Response: This is a rapid, short-term response to external stimuli like touch, vibration, or heat. It is a thigmotropic response, meaning it is triggered by physical contact. The purpose of this response is likely to deter herbivores by making the plant appear less appealing.
* Direction: The movement is not directional, it is simply a response to a stimulus, regardless of its origin.
Shoot Phototropism:
* Mechanism: The movement is caused by the uneven distribution of auxin, a plant hormone, on the shoot tip. When light shines from one side, auxin accumulates on the shaded side of the shoot, causing cells on that side to elongate faster than those on the lit side, resulting in the shoot bending towards the light.
* Response: This is a slow, long-term response to a directional stimulus, light. It is a phototropic response, meaning it is triggered by light. The purpose of this response is to maximize sunlight exposure for photosynthesis.
* Direction: The movement is directional, always towards the light source.
Key Differences:
* Stimulus: Sensitive plants respond to touch, while shoots respond to light.
* Mechanism: Sensitive plants use turgor pressure changes, while shoots use auxin distribution.
* Response: Sensitive plants exhibit a rapid, short-term response, while shoots exhibit a slow, long-term response.
* Directionality: Sensitive plant movement is not directional, while shoot movement is always towards light.
In summary, the movement of leaves in a sensitive plant is a rapid, temporary response to touch, while the movement of a shoot towards light is a slow, directional response aimed at optimizing light exposure for growth.
