448 



VENTILATING SYSTEM 



the thin and very convex inner walls of the subsidiary cells act as the 

 inner hinges. 



With regard to the contents of guard-cells, it may first of all be 

 stated, that these elements are almost always characterised by the 

 presence of chloroplasts, which often contain a large amount of 

 included starch. The protoplast is well-developed in other respects 



A 



B 



Fig. 168. 



Stomata with inner and outer hinges (H and H{). A. Chlorophytum comosum. 



B. Allium Cepa. 



also, and the nucleus often lies near the centre of the central wall, 

 especially in young guard-cells. 



Attention must next be directed to the mechanism of a stoma of 

 the ordinary type referred to above. In the first place, direct observa- 

 tion shows, that the pore of the stoma is opened or dilated when the 

 guard-cells become more strongly curved, and that it is closed or 

 contracted if the curvature of the guard-cells decreases. The width of 

 the pore thus depends directly upon the degree of curvature of the 

 guard-cells. The problem which we have to solve may therefore be 

 restated as follows : how does the anatomical structure of the guard- 

 cells lead, of mechanical necessity, to alterations in their degree of 

 curvature [under certain conditions] ? Von Mohl was the first to 

 approach this problem along correct physiological lines. He pointed 

 out, that when pieces of living epidermis are mounted in water, the 

 stomata open or become dilated, whereas immersion in a solution of 

 sugar produces the opposite effect. This simple experiment proved 

 that changes of turgor provide most, if not all, of the motive power in 

 the process of opening or closing. But it was reserved for Schwen- 

 dener to show, first, that these turgor changes take place mainly in the 

 guard- cells themselves ; secondly, that the opening and the closing of 

 the orifice are due to changes in the shape of the guard-cells; and thirdly, 

 that turgor-variations produce these changes of shape owing to the 

 peculiar distribution of thickened and unthickened areas in the walls 



