THE METABOLISM OF PLANTS. 323 



up in the stem, and, as a consequence, the former was found 

 to be electrically negative as regards the latter. Again, in 

 the experiments with stems which had been cut across, the 

 section was covered with moisture which had escaped from 

 the injured cells, and the cells in the immediate neighbour- 

 hood of the section were saturated with water ; hence the 

 diffusion-currents were more active at the point of the un- 

 injured surface touched by the one electrode than at the 

 section touched by the other, and consequently the former 

 was positive as compared with the latter. With regard to 

 Ranke's " true current," there can be little doubt that it is 

 susceptible of explanation in much the same way. Hermann 

 mentions that the artificial longitudinal section is often posi- 

 tive to the artificial transverse section, especially when the 

 organ consists for the most part of cells elongated in the 

 direction of its length. It might be suggested that this cur- 

 rent is due to some chemical difference in the cell-sap of the 

 cells, in contact with the electrodes on the longitudinal and 

 transverse sections respectively,, but Velten has shewn that 

 this is not the case. He prepared a piece of the stem of Sida 

 napcea which shewed the "true current," and he found that 

 both the longitudinal and transverse sections were strongly 

 acid. He then washed the longitudinal section with dilute 

 solution of soda until its reaction was slightly alkaline, and 

 in another instance he treated the transverse section in the 

 same way. On examination it was found that, in these pieces 

 of stem, With one acid and one alkaline surface in contact 

 with the electrodes, the current remained the same as when 

 both the surfaces were acid. 



We see that there is some reason for accepting the view 

 that the electrical currents which have been observed in plants 

 do not indicate a dissipation of the energy of the plant, but 

 are due to physical causes, and are even induced by the 

 apparatus employed for the purpose of detecting them. The 

 crucial test lies in the comparison of living and dead organs. 

 Ranke observed that parts of plants which had slowly died no 

 longer shewed the " true current," though irregular " false cur- 

 rents" could be detected for some time. Munk found that 



21 2 



