|MadSci Network: Botany|
I must admit that I am not familiar with the nail polish/tape technique you mention. I will assume that it strips off the epidermis, and this is what you are looking at under the microscope, rather than just a replica of the leaf's surface. If this is indeed the case, I could easily imagine that the exposed guard cells would become dehydrated and lose their turgor, thus appearing to close the stomates. Are you looking at the peel under a cloverslip, or simply on a slide exposed to the air. If the later, you might use a coverslip, and try adding different liquids, e.g. water and various concentrations of NaCl, to see how this affects the stomatal closure.
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