1 84 THE STOMATAL MECHANISM 



guard cells and by changes in the turgor of the epidermal cells, although 

 the former usually play a predominant role. In general an increase in the 

 turgor of the guard cells relative to that of the epidermal cells leads to a 

 widening of the stomatal aperture, and vice versa. The greater this turgor 

 difference, the wider the stomatal aperture. 



The mechanism of the efiect of changes in the turgor of the guard cells 

 upon the size of the stomatal aperture varies with the structure, form, and 

 position of the stomates. In one type of guard cell, found in many different 

 species of plants, the cell wall is thicker on the side bordering the stomatal 

 pore than on the side bordering the epidermal cells (Fig. 43). With an 

 increase in turgor the thinner walls of the guard cells are stretched more 

 than the thicker ; this causes the thicker-walled sides to assume a concave shape, 

 and results in the appearance of a gap — the stomatal pore — between the two 

 guard cells. Opening of the stomates typical of the grass and sedge families 

 (Fig. 42, C), appears to be due to swelling of the ends of the guard cells 

 thus separating the abutting walls of the middle portion of the two adjacent 

 guard cells. In the sunken stomates typical of conifers (Fig. 42, D), open- 

 ing of the stomates seems to result largely from a change in the shape of 

 the guard cells due to an increased turgor which is unaccompanied by any 

 appreciable stretching of the walls. These various types of stomatal mechan- 

 isms are discussed by Copeland (1902). 



The three principal factors which influence the opening and closing of 

 the stomates are: (i) light, (2) the internal water relations of the leaf, and 

 (3) temperature. 



I. Influence of the Light Factor in Stomatal Opening and Closing. — 

 The most familiar of all stomatal reactions is their response to light. Unless 

 other conditions, to be discussed later, are limiting, the stomates of most 

 species open when exposed to light, and close upon the failure of illumination. 

 Most commonly, therefore, the stomates are open in the daytime and closed 

 at night, although there are many exceptions to this statement. The sensitivity 

 of stomates to the light factor probably varies considerably according to species. 



Within limits the stomates appear to respond quantitatively to the amount 

 of light absorbed. Stomatal opening apparently will occur in all wavelengths 

 of the visible spectrum, although the influence of radiations in the red region 

 appears to be weaker than the influence of other wavelengths of the visible 

 spectrum (Sierp, 1933). 



Upon the cessation of illumination the stomates usually begin to close. 

 Generally this is a gradual process and, according to Stalfelt (1929) the 

 greater the quantity of light which has been absorbed during the course of 

 a day, the longer it takes for the completion of stomatal closure. 



