noun, plural: stomates
The stomates are actually the pores created by the swelling of guard cells to allow CO2 to enter into the leaf, which is a necessary reactant of photosynthesis. The water vapor and O2 are also allowed to escape via the pore. In order to form the pore, osmotic pressure draws water to increase the cells volume; this in turn causes the guard cells to bow apart from each other because the inner wall of the pore is more rigid than the wall on the oppostie side of the cell.
Stomates are present in all terrestrial plants (in sporophyte phase), except for the liverworts. Dicots usually have more on the lower epidermis than the upper epidermis whereas monocots usually have the same number on both sides. Plants whose leaves float in water have stomates only on the upper epidermis whereas plants whose leaves are completely submerged may lack a stomate entirely.