MECHANISM OF STOMATA. 269 



so that they may be fairly regarded as a part of the system for 

 aerating the plant. 



722. By reference to the structure of the more common kinds 

 of leaves (see Chapter III.), it will be seen that the terminations 

 of the delicate fibrils of the framework approach very closely 

 to the aeriferous spaces, and thus by the uninterrupted com- 

 munication between the minute fibrils in the root-system, the 

 stem- system, and the leaf-system of the plant, water which has 

 been absorbed by the roots is brought finally to the parenchyma 

 cells which surround the spaces under the stomata. If it 

 evaporates from the outer side of the wall of these cells into 

 the intercellular spaces, the water may make its escape through 

 the stomata. 



723. Stomata are not mere epidermal rifts having an aper- 

 ture of unvarying width. The guardian cells of a stoma are so 

 arranged with respect to each other and the proper epidermal 

 cells contiguous to them, that the width of the opening between 

 them can be increased or diminished upon certain changes in 

 the surrounding conditions. 



724. Mechanism of Stomata. In examining the mechanism of 

 stomata it is necessary to distinguish between their three parts 

 which are shown in a vertical section ; namely, (1) the anterior 

 groove, (2) the cleft, and (3) the posterior groove, which is 

 usually continuous with an intercellular space. It is plain that a 

 stoma is most widely open when the edges of the cleft are farthest 

 apart and the rim of the cup not closed. Hence an inspection 

 of the anterior face of a- stoma is not sufficient to show whether 

 the stoma is most widely open ; the width of the cleft itself must 

 be ascertained. 



725. In distinction from proper epidermal cells, the guardian 

 cells contain chlorophyll, and hence under the influence of light 

 can produce carbohydrates (see "Assimilation"). As might be 

 expected, the osmotic tension is different in these two groups 

 of cells. 



726. The following account, condensed from Strasburger, 

 shows the relations which the guardian cells sustain to those 

 around the stoma as regards the thickness of the walls. The 

 guardian cells are strongh* thickened on the upper and under 

 angles of the walls of their opposed faces, while elsewhere their 

 walls are relatively thin. At the cleft there are opposing 

 projections forming its edges. The opening and closing of a 

 stoma depend upon the difference in the thickness of the parts 

 of the walls. When the turgescence of the guardian cells 



