STOMATA PORES

Function

The gas exchange that occurs when stomata are open facilitates photosynthesis. Photosynthesis is the process by which plants convert sunlight into usable energy. During photosynthesis, carbon dioxide is taken in from the atmosphere through the stomata and oxygen is released as a waste product. Both photosynthesis and the gas exchange that powers it are essential to the plant's survival.
An unfortunate side effect of the stomata opening is that it allows for water loss. Unlike you and me, plants do not need to sweat to cool off and prefer to keep their water inside; however, because the gas exchange of photosynthesis is so vital, some water loss through stomata is necessary. This process of plant water loss is called transpiration.
Although transpiration cannot be avoided, plants can minimize their water loss by controlling how wide their stomata are open, as well as what time of day they are open. Opening stomata when the surrounding air is more humid means that less water will evaporate from the plant leaves, but opening them when temperatures are warmer means more evaporation will occur. Likewise, if a plant is already dehydrated, it may close its stomata to prevent further water loss.

Structure

A stoma is the opening on a plant leaf, but there are specialized cells surrounding each stoma that control how open or closed it is. These specialized cells are called guard cells, and they are triggered by a number of factors, such as sunlight, humidity, temperature, and internal plant chemistry.