The opening on the underside of a leaf that prevents water from evaporating is called a stoma (plural: stomata).

Here's how they work:

* Structure: Stomata are tiny pores surrounded by two specialized guard cells.

* Function:

* Gas Exchange: Stomata allow for the exchange of gases like carbon dioxide (needed for photosynthesis) and oxygen (a byproduct of photosynthesis).

* Water Regulation: The guard cells can open and close the stomata, controlling the rate of water vapor loss through transpiration. When the plant needs to conserve water (like during hot, dry conditions), the stomata close.

So, while stomata are essential for gas exchange, they also play a crucial role in preventing excessive water loss.