28 RESEARCHES ON IRRITABILITY OF PLANTS 



tin-foil — separated by a sheet of non-conducting material, 

 such as mica or paraffined paper. The capacity of the 

 condenser is increased by enlarging the effective area of 

 the plates. The diagram in fig. 9 illustrates this mode of 

 excitation. By increasing the number of cells, the charging 

 e.m.f. may be increased until a suitable value is obtained 

 which is efficient for excitation. This will depend on the 

 excitability of the plant-specimen. About 2 volts charging 

 •5 microfarad will in general be found sufficient, k is a 

 special spring-key by which the condenser may be charged 



Fig. 9. — Direct stimulation by condenser discharge ; c, condenser, k, 

 key. (a) intra-electrodal and (b) indirect extra-electrodal mode of 

 stimulation. 



or discharged. The plant to be excited is included in the 

 electrical circuit. When k is pressed down, the condenser 

 is charged, the instantaneous charging current passing in 

 one direction. The upper arrow in the diagram shows 

 the direction of this charging current. When the key is 

 released, it springs back and discharges the condenser. 

 The instantaneous discharge current now flows in a reverse 

 direction (fig. 9). 



The electrical connections with the plant are diagra- 

 matically shown, in this and other figures, by two lines. 

 In practice the connections have to be made by means 

 of thread moistened in dilute saline solution. In certain 

 experiments it is necessary to avoid complications arising 

 from electrolytic polarisation. In these it is advisable to 



