The stomata you view on the surface of the leaves are used in gas exchange and the guard cells regulate this exchange. What do you think the response of the guard cells will be if you leave the leaf under the microscope lamp for too long?

They will collapse and shut off the stomatal pore

Explanation:

The guard cells are regulated by the presence of water. When water is present, they become turgid and open up the stomatal pore and when water is inadequate, they become flaccid, collapse and close up the stomatal pore as a result.

If the leaf is left under the microscope for too long, there will be loss of water by evapotranspiration and the guard cell will become flaccid and collapse as a result and the stomatal pore will become closed.