Answer: The portal where oxygen leaves and carbon dioxide enters the leaf.
Explanation:
The structure in the picture is a leaf and the space indicated by the arrow is called the Stoma. It is located under the leaf as shown in order to reduce the loss of water through transpiration.
The primary function of the Stoma is to allow carbon dioxide to enter the cell which will be used for photosynthesis. When the process is complete and oxygen is produced as a by-product, it leaves the leaf through the Stoma as well.
