Final answer:
Stomata are the plant organs responsible for controlling water vapor loss.
Step-by-step explanation:
The plant organ that opens and closes to control water vapor loss is the stomata.
The stomata are small structures found on the surface of leaves and stems of plants. They consist of two guard cells that surround a small opening called the stomatal pore. The main function of the stomata is to regulate the exchange of gases, including water vapor, between the plant and its environment.
When the plant needs to conserve water, the guard cells close the stomata, reducing the loss of water vapor through transpiration. Conversely, when the plant needs to release excess water vapor, the guard cells open the stomata, allowing water vapor to escape.
Learn more about Plant physiology