Why is it necessary to leave the green leaf tube in the dark?

Leaving the green leaf tube in the dark is necessary to prevent photosynthesis from occurring. In the absence of light, the plant cannot produce glucose or oxygen through photosynthesis, allowing for the assessment of the plant's stored starch. This helps to demonstrate the role of light in photosynthesis and the importance of chlorophyll in capturing light energy. Additionally, it ensures that any changes observed in the leaf are solely due to the absence of light rather than ongoing photosynthetic activity.