Stomata 

Stomata are the tiny pores on the bottom-side of a leaf. A single hole is called a stoma, and it opens and closes.

Stomata control the exchange of gasses in and out of a plant. A plant needs to take in CO2 (carbon dioxide) and release O2 (oxygen). These gasses are transported in and out through the stomata.

Guard cells control the opening and closing of pores in the leaf. When the guard cells swell up, the pore closes. When the guard cells shrink, the pore opens.