Control of Transpiration 



53 



mouth open, but with their two ends abutting against each other 

 and pressing firmly with their backs against the surrounding tis- 

 sue of the leaf, 3-4 ; B is a cross -section of these cells along the 



M 



Fig. 6. Diagram showing the mechanical action of guard cells in opening and 

 closing breathing pores. The square shows the area of under side of leaf 

 containing an average of 209,000 breathing pores or stomata. 



line 1-2 ; while C and D are corresponding views with the breath- 

 ing pore closed. It will readily be seen that if the water holding 

 the two cells in A and B rigid and distended partially escapes 

 from them, their thin walls will then fall down and take the 

 positions shown in C and D, where, as no displacement can take 

 place in the directions away from the opening on account of the 

 surrounding tissue, the walls must advance toward each other, 

 more or less completely closing the aperture between them, as 

 shown at C and D. Then, too, when the cells again become dis- 

 tended and turgid, the pressure will tend to force them to take 

 the circular outline shown in section at B, and as the back wall 

 of the two is fixed to the tissue so as not to be able to move, 

 nearly all of the motion takes place upward and downward, and 

 this pulls the two faces which are not fixed away from each other 

 and widens the stoma or pore. It must, of course, be kept in mind 

 that the shape of the actual guard cells varies in detail in many 

 ways from the diagram given, and that we have here only intended 

 to illustrate the mechanical principle involved in their opening 

 and closing. 



We see, then, that not only is water a very important sub- 



