398 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY 



to those further away. The reaction extends for a limited distance, for Kunkel 1 

 could observe no difference of potential on a previously isoelectric stem when the 

 electrodes were placed 5 and 6 cms. away from the injury. When a piece is cut out 

 of a stem, the two cut surfaces are isopotential and the greatest difference of potential 

 exists between the median point of the stem and either cut surface. Right and 

 left of the median line isopotential zones exist, the connexion of which, by an 

 external circuit, produces no current. The distribution of potential is therefore 

 exactly the same as on an isolated cylindrical muscle, in which it has been more 

 deeply studied 2 . 



If the epidermis is removed from the segment of the stem, the current is 

 immediately reversed according to Ranke and Velten 8 , flowing from the transverse 

 section to the longitudinal surface in the external circuit. A similar reversal may 

 take place, according to Hermann, on pieces of stem in which the epidermis remains 

 uninjured, and which showed at first similar currents to those in muscle. 



There are exceptions to these rules, and possibly more will be found in the 

 future. Thus Ranke * found that on the petiole and peduncle of Nymphaea alba 

 the current passed from the transverse surface to the longitudinal epidermal one both 

 before and after the epidermis had been removed, while Velten 6 found that the 

 current directed from the longitudinal surface to the transverse one persisted after 

 the removal of the epidermis. 



The current of injury is produced instantly, so that if the electrodes are laid on 

 the stem and an incision made near to one of them, an immediate deflection is 

 produced in the galvanometer. It is, however, uncertain whether the reaction is 

 purely one of physical chemistry or is due to a vital action such as that which leads 

 to the closure of the leaflets of Dtonaea. In ordinary tissues the electrical changes 

 might form the first indication of the physiological reaction leading to an increased 

 activity of respiration and an enhanced production of heat. It is also unknown to 

 what extent the electrical changes are connected with the gradual progress and 

 development of the wound-reaction. The electrical variations are produced when 

 neither electrode touches the cut surface, as well as when it is at once washed with 

 water. Kunkel 6 found that local bending also produced a negative variation at the 

 part affected, and it has yet to be determined whether this variation and the variation 

 due to injury are produced in the same way. If so, then the injury and death of 

 cells would not form an essential condition for the production of the ' injury ' current. 

 In all cases it must be remembered that the removal of the epidermis decreases the 

 previous electrical resistance, and furthermore that electrical stimulation may con- 

 siderably increase the conductivity more especially of young and highly protoplasmic 

 tissues such as the cambium and apical meristem. 



1 Kunkel, Arb. d. bot. Inst. in Wurzburg, 1878, Bd. II, p. 6. 

 a Cf. Biedermann, I.e., p. 275. 



3 Ranke (1. c.) calls the current led off from an uninjured epidermal surface the false, and that 

 from the injured epidermal surface the true plant-current. 



4 L.C., p. 197. s L. c., p. 291. 

 6 L.c., 1878, p. 7. 



