101 G 



ridtJ-es are not tliose of the üuanl-cells but of tlie subsidiary cells. 



Wlieu a transverse section througli the leaf, somewhere about the 

 line (/ of F.ig. 1, cuts the ston?a parallel to the long axis, a view 

 is obtained corresponding with Fig. 7. From above downwards the 

 much thickened and cuticularised outer-wall, the lumen of the sub- 

 sidiary cell, the wall dividing the subsidiary cell from the guard- 

 cell, the lumen of the latter and the thick ridge of cellulose here 

 follow each other. If on the other hand the razor follows the line ^^ 

 then as in Fig. 8, we see from above dowjiwards the cuticularised 

 edge which adjoins the "Vorhof", the lumen /)f the guard-cell, the 

 wall separating the guard-cell from the subsidiary cell, and the lumen 

 of the subsidiary cell. In similar longitudinal sections through «he 

 stoma, the subsidiary cell can be seen either above or below the 

 guard-cell, according to the direction in which the section is made. 

 In Fig. 8 we may see also the peculiar shape of the guard cell, 

 somewhat like that of a dumb-bell, which can also be made out 

 by combining the sections of Fig. 4 and Fig. 5. 



If we now wish to get an understanding of the functions of this 

 complicated apparatus, we must assume that the outer slit formed by 

 the ridges of the cuticle can be considerably narrowed and widened. 

 Perhaps the inner slit, bounded by the thick edges of cellulose 

 can also be narrowed and widened, though in all probability only 

 to a slight extent. Variations of turgor in the subsidiary cells, must, 

 as is at once clear from the structure, exert an influence on the 

 width of the outer slit. 



The central slit (Centralspalte) is doubtless also capable of varia- 

 tion in width; it seems however that it is never wholly closed, but 

 that a small opening always remains, as represented in Fig. 2. 



As long as the subsidiary cells are turgid, they otfer resistance to 

 the pressure of the guard-cells ; they cannot curve as strongly and 

 open the central slit as widely as would be the case if no counter- 

 pressure were exerted by the subsidiary cells. 



When in consequence of transpiration, the turgor of the subsidiary 

 cells decreases before that of the guard-cells, a stronger curvature of 

 the latter and widening of the central slit must take place. 



It is likely that, when the plant first withers, the turgor indeed 

 decreases earlier in the subsidiary cells than in the guard-cells, because 

 the guard-cells are almost completely surrounded by the subsidiary- 

 cells and the small part of the wall of the guard-cells adjoining the 

 opening is rather strongly cuticularised. I believe that in the first 

 place the chlorophyll-containing parenchyma loses water by trans- 

 piration, that these cells abstract water from the epidermal-cells and 



