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.
The main gas exchange surfaces in plants are the spongy mesophyll cells in the leaves. Leaves, of course, have a huge surface area, and the irregular-shaped, loosely-packed spongy cells increase the area for gas exchange still further.as exchange takes place at a respiratory surface-a boundary between the external environment and the interior of the body. For simple unicellular organisms the respiratory surface is the cell membrane, but for large organisms it is part of specialised organs like lungs, gills or leaves.