Plants regulate the rate of transpiration by controlling the size of the stomatal apertures. Other studies have explored the relationship between drought stress and stomatal conductance. A number of models of stomatal conductance exist. All these factors work to have a net decrease of stomatal conductance. Particulate matter can clog stomatal openings of plants and interfere with photosynthesis functions. It inhibits cell division, promotes seed maturation, and dormancy, and promotes stomatal closure. This required an increase in stomatal density by 100 times to maintain rates of photosynthesis. The loss of solutes makes the cell flaccid and closes the stomatal pores. The fact that they lack chloroplasts has made them valuable to researchers investigating stomatal function. Anion channels were identified as important controllers of stomatal closing. For example, it enabled the discovery of intracellular events that precede stomatal closure. They are produced in pairs with a gap between them that forms a stomatal pore. Plants take in through stomatal pores on their leaves. A key hormone regulating stomatal opening and closing is abscisic acid (ABA). Sodium can potentially replace potassium's regulation of stomatal opening and closing.