Stomata: Structure, Function and Microscopy

A pore with a purpose

A leaf needs carbon dioxide from the air for photosynthesis, but its surface also loses water. Tiny pores called stomata allow gases to pass between the air and the spaces inside the leaf. Stoma means one pore; stomata means more than one. Each pore is bordered by a pair of guard cells. The guard cells change shape to open or close it.

When a stoma is open, carbon dioxide diffuses into a photosynthesising leaf and oxygen produced by photosynthesis diffuses out. Diffusion is the net movement of particles from higher to lower concentration. Water evaporates from moist surfaces inside the leaf, and water vapour then diffuses out through the open pore. This loss of water vapour is part of transpiration. Closing the pore reduces water loss, but also limits the entry of carbon dioxide: the same opening serves both gas exchange and water loss.

Seeing stomata with a light microscope

Stomata are too small to inspect clearly with the unaided eye. A practical way to observe their structure is to make an impression of a leaf surface. Paint two thin layers of clear nail varnish onto the surface, allowing the varnish to dry. Press clear sticky tape over the varnish, then peel the tape off with the impression attached. Stick the tape flat onto a microscope slide.

Place the slide on the stage of a light microscope. Start with the lowest-power objective, bring the impression into focus, then change to a higher-power objective and use the fine focus to see more detail. Look for a small pore between two guard cells; nearby outlines are other cells of the leaf surface. The varnish impression shows the surface pattern rather than a section through the whole leaf. You can repeat the method on the other leaf surface to compare what you see: stomata are often more numerous on the lower surface, but their distribution is not identical in every leaf.