AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
The plant is kept in the dark for three days so that all the previously stored starch gets completely used up (exhausted). This ensures that when the plant is later placed in sunlight, any starch found in the leaf has been freshly produced by photosynthesis, giving a clear and reliable result.
Source: Life Processes, Chapter 5, Activity 5.1
Examiners expect two points: (1) dark conditions stop photosynthesis so existing starch is consumed, and (2) this gives a "clean" starting point so results are not confounded by pre-existing starch. Both points together earn 2 marks. Don't forget — starch is the stored form of carbohydrate produced during photosynthesis, as stated in section 5.2.1.