Assertion : According to Scarth, opening and closure of stomata is controlled by $\mathrm{pH}$ value.
Reason : Low $\mathrm{pH}$ value favours opening of stomata and higher $\mathrm{pH}$ value closure of stomata.

Each stomata is surrounded by two distinctive kidney-shaped epidermal cells known as guard cells. These stomata are turgor operated valves and thus the opening and closing of the stomata are determined by the turgor changes of the guard cells. factors that govern the opening and closing of stomata are presence and absence of light, high or low ph, reduced or high carbon-di-oxide levels, as well as the availability of water. Scarth (1932) proposed the starch-sugar interconversion to explain how pH changes take place in guard cells.
Stoma in the leaves is surrounded by the guard cells. Opening and closing of stomata can occur due to change in the turgidity of the cells. There is a huge change in the pH of the cells. . Low pH value favours opening of stomata and higher pH value closure of stomata.