We test for the presence of starch in leaves in order to determine that photosynthesis has occurred. Glucose - the product of photosynthesis - is rapidly converted into starch for storage in the chloroplast and cytoplasm.
When investigating whether a condition (or a factor) is necessary for photosynthesis is to occur we need to remove all traces of starch in leaves so that we can provide evidence that photosynthesis takes place during the experiment i.e. in the presence of this factor and not in its absence.
If starch is already present at the beginning of the experiment the results will not be accurate. Hence, it is important to start off with a plant without starch. This is accomplished by de-starching the leaves of a plant.
