Plants use pores called stomata for gas exchange. Carbon dioxide (CO2) enters, and oxygen (O2) produced in photosynthesis exits the plant through the stomata. Water molecules also escape through the stomata in the process of transpiration, which is why guard cells are important. When a plant is becoming dehydrated, guard cells close to prevent further water loss. Many plants that live in very arid environments only open their stomata at night to minimize water loss.
Obtain a leaf and paint a thick layer of clear nail polish on one surface. Allow the polish to dry completely. Place a piece of clear tape over the nail polish patch. Peeling the tape off the leaf will also remove the nail polish. Tape the nail polish to a slide and trim any excess tape with scissors. View the leaf impression with the microscope, under high power. Are the majority of the stomata open or closed? What can this tell you about the plant?
Focus on one stoma and observe the guard cell impressions. Repeat this procedure with the other leaf surface. How does the number of stomata observed compare with the number from the other surface?