160 PHYSIOLOGY OF NUTRITION. 



The projecting ends of the electrodes are fixed to the stands. 

 The two ends of the main glass tube are fitted with rubber 

 stoppers, through which pass glass tubes. If now we place in the 

 moist chamber a pea seedling 10-15 cm. long, and lead through 

 the apparatus a slow current of air, saturated with aqueous 

 vapour by passage through a small U-tube filled with wet glass 

 wool, we get, according to Haake, a very large deflection of the 

 galvanometer when one electrode is placed on the neck of the 

 root, the other on the stem of the seedling. The deflection be- 

 comes however slight if now, for about a quarter of an hour, we 

 pass a slow stream of Hydrogen through the apparatus. The 

 Hydrogen is purified by passage through a solution of Potassium 

 permanganate, and saturated with aqueous vapour by being led 

 over moist glass wool. Renewed access of air again leads to con- 

 siderable deflection of the galvanometer. In these experiments 

 care must always be taken to keep the plants in the dark, so as to 

 prevent assimilation. 



If, therefore, we deprive plant structures of Oxygen, we bring 

 about a change in their electric condition, and this justifies us in 

 concluding that metabolism and respiration must be regarded as a 

 cause of the differences of potential to be observed in plants. It 

 is true, killed plants {e.g. pea seedlings killed by steam) also 

 exhibit differences of potential, but these do not undergo any 

 marked change on withdrawal of Oxygen, which proves that they 

 owe their origin to causes (chemical changes in the dead struc- 

 tures) quite different from those taking place in the living plant. 



In examining the results of our observations, it is very important 

 to bear in mind that at present we are only able to demonstrate 

 generally the existence of differences of potential in plants. As 

 to their true individual value we know nothing. The cells may be 

 compared with small voltaic elements, but the galvanometer indi- 

 cates only the difference between the strengths of the currents 

 leaving the different cell complexes brought into contact with the 

 electrodes. Even if the galvanometer no longer indicates any 

 current at all, electric actions might still be going on ; the currents 

 sent through the galvanometer in opposite directions from the 

 two points might be equal. 



1 See Haake, Flora, 1892, and Kunkel, Arbeiten d. lotan. Instituts in Wiirz- 

 burg, Bd. 2. In Kunkel, see also observations on differences of potential in 

 resting and mechanically stimulated leaves of Mimosa, and on the diminution 

 of current (negative variation) exhibited on stimulation. 



