This is an image of a activity to show that sunlight is necessary for photosynthesis.
Here the uncovered part of the leaf of plant will prepare food and this food should be transport throught the plants even in covered part of leaf (marked in image) through vascular tissue.
So if food is transporting in that covered part also then how there is not any stored starch there?

This activity is exclusively performed to study that sunlight is necessary for photosynthesis. Plants prepare food in a very short span of time and the food thus prepared is transported to other parts of the plant through phloem. The leaf used in the experiment has some part of it covered and simply this part will not perform photosynthesis. Even if it gets some amount of food through translocation of prepared food, it will not show up in the starch test as we perform this activity in a particular time interval that does not allow transportation of food. 

Hope this helps.
Regards.