DIFFERENCES IN POTENTIAL 



103 



be touched with the electrodes. Now pass a slow stream of air 

 through the chamber, and a movement of the electrometer may 

 be noticed. Connect a hydrogen generator back of the moist 

 chamber and pass a stream of well washed hydrogen (in per- 

 manganate of potassium) through the moist chamber and into 

 the electrical chamber. As the hydrogen displaces the oxygen 

 the difference in potential seen at first will disappear, leading to 

 the inference that it was caused by oxidation, though it is by no 

 means to be considered as absolute proof. 1 



FlG. 45. Electrical chamber for testing relation of oxygen to differences in elec- 

 tric potential. After Haacke. 



159, Differences in Potential Between Illuminated and Non-illu- 

 minated Portions of a Stem. Set up a preparation as in the pre- 

 vious experiment and note the difference in potential between 

 the regions in contact with the electrodes. Cover the entire 

 tube with cloth, or some device for effectively excluding light, and 

 move to a position to receive the direct rays of the sun. Un- 

 cover one end of the tube and allow the light to fall upon the 

 region in contact with one electrode. Cover and a few minutes 

 later repeat allowing the rays to strike the region near the other 

 electrode. A current will be found to set in from the illumi- 

 nated to the darkened area. 2 



1 Haacke, O. Ueber die Ursachen elektrischer StrSme in Pflanzen. Flora, 

 75 : 455- 1892. 



2 Waller, A. D. The electrical effects of light upon green leaves. Abs. Science, 

 12: 377. 1900. 



Klein, B. Zur Frage ueber die elektrischen Strome in Pflanzen. Ber. Deut 

 Bot. Ges. 16: 335. 1898. 



