450 VENTILATING SYSTEM 



the ventral wall to swing backwards and forwards around its median 

 plane, so that it bulges towards the pore when the stoma closes. As 

 Schwendener has pointed out, the two thickened ridges may be 

 compared to the stiff boards of a portfolio, while the thin median strip 

 corresponds to the flexible back. Finally, the stomatic pore can be 

 closed more tightly, if the membranes which become opposed to one 

 another are thin and pliant, instead of being thickened and hence 

 comparatively rigid. 



In curving under the influence of increased turgor, the guard-cells 

 must, of course, overcome the resistance of the adjoining epidermal 

 cells, which are themselves in a turgid condition. If this resistance is 

 diminished for example, at the edge of a severed strip of epidermis 

 where the epidermal elements are laid open the curvature of the 

 guard-cells increases beyond the normal amount. 



When a stoma is open, the internal tension of its guard -cells must, 

 of course, exceed the osmotic pressure which prevails in the ordinary 

 epidermal elements, and in the subsidiary cells, if such are present. 

 According to Schwendener, the pressure in guard-cells amounts, under 

 certain stated conditions, to about 5 to 10 atmospheres. 



As already stated, the type of stoma which has just been 

 described is the most widely distributed of all. Its action depends 

 upon the fact, that the thickened strips are only developed on the 

 ventral walls of the guard-cells, and are thus arranged asymmetrically 

 with reference to the plane which passes through the middle of the 

 guard-cell, at right angles to the surface of the leaf. In a considerable 

 number of Dicotyledons, however, the walls of the guard-cells are 

 thickened all round, in such a way that the cavities are reduced to 

 narrow transverse slits. Here the massive semi-cylindrical thickened 

 strips of the wall are distributed symmetrically on two sides of the 

 median plane of the guard-cell. In these circumstances there is no 

 appreciable difference between the dorsal and the ventral walls, with 

 regard to the extension produced by the rise of turgor. Where 

 stomata of this second type are functional at all, which is not always 

 the case, the curvature of the guard-cells obviously cannot be brought 

 about in the same manner as it is in the instance first described. It 

 is possible that the guard-cells tend to elongate when their internal 

 tension increases, but that they are forced to bend aside owing to the 

 resistance of the epidermal cells to which their ends are attached, just 

 in the same way as a [flexible] vertical column will be deflected to 

 one side if too heavy a load is applied to the upper end. In order, 

 however, that an outward curvature may result in every case, the 

 guard-cells must be slightly curved in the same sense, even when they 

 are not distended. Guard-cells of this symmetrical type occur in the 



