Question

a leaf covered with Vaseline is tested for starch

Answer 1

Covering the leaves with Vaseline will close the stomatal opening present on leaf epidermis. Stomata helps in gaseous exchange and water evaporation or loss from leaf surface . Hence two major process of plant survival will be affected, which are :
(i) Transpiration {loss of water from leaf surface}
(ii) Photosynthesis / Respiration:

Hope it helps u.

Answer 2

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This is because plant will not get oxygen for respiration. it will not get carbon dioxide for photosynthesis. The chemical composition of Vaseline contains the residue and compounds of petrol and its product which is very harmful for the plants and these composition can dry up a plant if applied on their leaves.