What is meant by photonasty?

Ans : The non - directional movement of a plant part usually the petals of a flower in response to light is called photonasty. 

Examples : A dandelion flower opens up  in the morning in bright light but closes in the evening when the light fades and it gets dark. Whereas a moonflower behaves exactly  opposite. The petals of moonflower close during the day when there is bright light but open up at night when it  is dark and there is no light.