Question

Why should we destarch a leaf before conducting an experiment to show that lightvis necessary for photosynthesis?

Answer 1

A plant is destarched before performing any experiment in order to prevent photosynthesis. 
In most of the experiment related to photosynthesis presence of starch is detected if plant is not destarched leaves give positive starch test due the presence of already stored starch. We can not confirm it is formed during photosynthesis.

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Answer 2

the plant is discharged