THE WORK OF LEA VES 



211 



each other, and is secured at the bend by a clothes-pin. 

 The free ends are connected to the arms of a glass 

 U-tube which have been bent near together. The end 

 of the U-tube is connected by a stout piece of rubber 

 tubing with a bicycle pump. The connections are all 

 tightly tied with twine or elastic bands, and air is 

 pumped in. As the pressure increases, the outer, thin- 

 ner walls of the rubber guard-cells bulge outward, 

 causing them to curve and leave the stoma or space 

 between them open ; when the pressure is released 

 they collapse and close it. The degree of 

 opening or closing is great or small accord- 

 ing to the elasticity of the rubber, the 

 rigidity of the reinforced wall, etc. 



If a U-tube is not at hand, use in place 

 of it two short glass tubes inserted in a 

 rubber stopper (enlarged a little at the end 

 to prevent them from slipping out), as 

 shown in Fig. 123 ; the cork is inserted in 

 a piece of glass tubing, at the other end 

 of which is a similar cork pierced with a 

 glass tube which is connected with the 

 pump. A wire may be passed over both 

 stoppers to hold them firmly in place- 



The danger of drying up is the most serious with 

 which the plant has to contend, and nothing affects 

 the health of a plant so quickly as lack of water. 



Plants suffering from dryness show very character- 



123. Modifica- 

 tion of part 

 of the appara- 

 tus shown in 

 Fig. 122. 



