THE WORK OF LEAVES 



209 



draws water from the neighboring cells and causes the 

 guard- cells to expand and open the stomata. When, 

 on the other hand, unfavorable conditions come, the 

 sugar -making ceases, and if the water supply begins to 

 run low the guard- cells collapse and close the stomata. 

 The manner in which the opening and closing is 

 effected may be explained by reference to Fig. 120, 

 which shows a stoma of Iris in both the closed and 

 the open position. The guard- cell (as seen in cross- 

 section, Fig. 121) has a thick wall on the side 

 toward the stoma and a thin wall on the opposite side. 

 According to mechanical laws, when such a structure 

 expands from internal pressure (due to the absorption 

 of water) the thinnest or weakest side must bulge 

 outward, causing the cell to curve (as shown by the 

 light lines in Fig. 120) . When the guard-cells expand, 



121. 



Diagram of a stoma of Iris (seen in section) showing guard-cells 

 and neighboring cells of epidermis. 



