Stoma

Plants inhale carbon dioxide and exhale oxygen through tiny closable holes normally found on the underside of leaves. These holes are called stomata (singular stoma).

Colorized electron microscope image of tomato leaf stoma (Photohound) Public Domain

Stoma structure

On either side of each stoma are two guard cells. These guard cells use osmosis to fill themselves with water (or release it) to close (or open) the stomata. Each evening, when photosynthesis has halted, the stoma close up shop for the night and reopen in the morning. As temperatures rise and water becomes more critical, the stoma will close. Each stoma is connected to a series of air spaces within the plant. Air diffuses through these spaces, delivering carbon dioxide and releasing oxygen. In effect, plants hold their breath when it is too hot for them and they do not have enough absorbed water - no wonder they wilt!

A stoma in cross section showing guard cells (Yvan Lindekens) CC BY-SA 3.0

Stomata control water flow

One really amazing aspect of stoma behavior is how they control water flow within a plant. Chemically, water will always move from areas of high humidity to areas to low humidity. When a stoma opens, it creates a bubble of high humidity. This exhaled moisture evaporates or is pulled away. Due to surface tension, more water is pulled out of the plant. This action works its way down to the roots, which then pulls water from the soil! This is how plants absorb water from the ground!