Why is the closing of a dandelion flower at dusk (when it gets dark), not a tropism?

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Solution

The closing of a dandelion flower at dusk is not a tropism because it is a non-directional movement in response to the intensity of light. On the contrary, the tropic movement is the movement of plant parts towards the light. It does not depend upon the intensity of light.

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Q. Dandelion flowers open the petals in bright light during the daytime but close the petals in dark at night. This response of dandelion flowers to light is called :
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(c) chemotropism
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