Murfreesboro is a city in Pike County, Arkansas, United States. The population was 1,764 at the 2000 census. The city is the county seat of Pike County. Crater of Diamonds State Park is located just to the south. The crater is an eroded lamproite pipe where diamonds were discovered in 1906. 100,000 were found by 1933, and visitors to the park may still take away whatever they can find.
The guard cells surround each stoma in a plant cell. They regulate  the opening and closing of stomata to facilitate gas exchange and  control transpiration in plants.
Light is one environmental signal for guard cells. Guard cells usually keep stomata open during the day and closed at night because during the night there is no sunlight for photosynthesis. If the stomata were open at night, very little gas exchange would occur, but water loss would still continue. Temperature also signals guard cells.