The are openings in the epidermis of stems and leaves and are used for gas exchange with the atmosphere.

The structures you're referring to are called stomata. They are tiny openings or pores, primarily found on the under-surface of plant leaves and stems. Here's how they work:

1. Stomata are surrounded by two specialized cells called guard cells that control the opening and closing of the stomata.

2. When the guard cells are filled with water, they swell, causing the stomata to open. This allows for the exchange of gases.

3. Carbon dioxide, which is essential for photosynthesis, enters the plant through these open stomata.

4. At the same time, excess oxygen produced during photosynthesis is expelled from the plant through these same stomata.

5. In addition to this, water vapor is also lost to the outside atmosphere through these stomata, a process known as transpiration.

6. When the guard cells lose water, they shrink, causing the stomata to close. This helps prevent excessive water loss during times of high temperature or low water availability.

So, in summary, stomata are crucial for a plant's processes of photosynthesis, respiration, and transpiration.