A cell in the outer covering of a plant's body that can change shape in order to open or close a pore

The cell you're referring to is known as a guard cell. Guard cells are specialized plant cells in the epidermis of leaves, stems and other organs that are used to control gas exchange. They are produced in pairs with a gap between them that forms a stomatal pore.

Here are the steps of how they function:

1. Water enters the guard cells: The process begins when light stimulates the guard cells to take in potassium ions. This causes water to follow by osmosis, making the cells swell.

2. Stomata open: As the guard cells take in more water, they begin to curve. This opens the stomatal pore, allowing carbon dioxide to enter and oxygen and water vapor to exit.

3. Water leaves the guard cells: When it's dark, potassium ions are pumped back out of the guard cells. Water follows by osmosis, causing the cells to shrink.

4. Stomata close: As the guard cells lose water, they become less curved. This closes the stomatal pore, preventing the loss of water vapor and the entry of carbon dioxide.

So, in summary, guard cells can change shape to open or close stomatal pores, controlling the gas exchange of the plant with its environment.