How does carbon dioxide enter plants?

All gas exchange in a plant takes place in it's stomata. Stomata are microscopic openings, kind of like little mouths, that open and close and take in CO2 and transpire O2 at the discretion of something called guard cells. The only job these guys have is to tell the stoma when to open and close based on the conditions around it. Basically the entire plant knows what to do based on what these guard cells learn. If temperatures are high and humidity is low, the stoma open all the way up to increase transpiration, which means the plant can take up more water to make up for the aired climate. If the humidity is much higher the stomata can close a little to reduce transpiration because there is plenty in the air.