190 



EXPERIMENTS WITH PLANTS 



109. Details of piston of the 

 air - pump shown in Fig. 

 108: (r) rubber disk, (d) 

 metal disk, (w) washer, 

 (n) nut. 



disk (d) which supports the rubber disk (r) . It has 

 the advantage of allowing a quick change of the 

 rubber. Through a small hole about four inches above 

 the rubber passes a 



stout piece of piano 



wire (or any steel wire 



suitable for springs) , 



which is bent as shown 



in the figure and has its 



ends passed through an- 

 other hole about an inch 



above the rubber. When 



the piston is drawn up, 



the wire springs out 



and prevents it from slipping back. 



When the apparatus is ready, place a 

 IDS. A* -pump leaf in the chimney and cover it with water. 



with auto- __ ,, . . n .. , 



rrange- Force the piston down slowly until the 

 water stands half an inch or so above it, 

 and then pull it up and secure it in place. 

 Does air issue from the leaf? If so, at what points? 

 Notice especially whether more air issues from the 

 upper or from the lower surface. Allow it to remain 

 until all the air is drawn out, making another stroke 

 with the piston if necessary. Notice the appearance 

 of the leaf when injected with water. Explain its close 

 resemblance to a leaf which has been boiled. 



If we prevent air from entering the leaf, how will it 



ment to pre- 

 vent the pis- 

 ton from slip- 

 ping back. 



