Why is oil added to the layer of water during the test for transpiration?
When plant is submerged into water, water is lost both through the plant surface, via transpiration, and via evaporation from water surface in container. If one wants to study transpiration only, he should block the loss of water from container. It is possible by pouring oil over water. It floats on the surface of water and prevents from water evaporation. Thus, water is lost only via transpiration, which is studied.