Plants respire through microscopic pores in the underside of their leaves; these pores are called stomata. Plants can open or close their stomata to regulate the amount of gas exchange that takes place.
There are three kinds of gases that plants release through their stomata: carbon dioxide, oxygen and water vapor. Each of these gases is a byproduct of a process essential to the plant's survival.
Plants produce carbon dioxide through cellular respiration. The amount of CO2 they release, however, is much less than the amount of CO2 they consume through photosynthesis. Photosynthesis releases oxygen gas as a byproduct and the oxygen diffuses out through the stomata of the leaves. Plants also lose water vapor through their stomata; this process of water loss is called transpiration.
Transpiration is essential to plants; in combination with the high surface tension of water, it creates negative pressure that draws water up the plant's stem and from the roots. Although it decreases their rate of growth, many plants will nonetheless close their stomata in response to stress caused by drought to avoid drying out.