POLAR EFFECTS OF ELECTRICAL CURRENT 207 



then the arrival of excitation at the pulvinus will be quicker 

 when the kathode is near and later when the kathode is 

 distant. It is clear from the records that this is what 

 obtains ; excitation is earlier when the kathode is proximal 

 and later when the kathode is distal. We find moreover 

 that the difference of time between the moments of arrival 

 of excitation in the two cases is represented by six spaces, 

 each of '05 second. Hence the delay in the second case 

 is equal to '3 second, due to transmission through the 

 additional distance of 5 mm. The velocity of transmission 

 is therefore 16 mm. per second, which we have seen is the 

 average velocity of transmission through the petiole of 

 Mimosa. 



After obtaining these characteristic effects with Mimosa 

 under feeble electric currents, we have still to find out 

 whether such effects are peculiar to this plant or whether 

 they are universally present. To answer this question we 

 have to experiment with every species of plant that happens 

 to be provided with a motile indicator. After this has been 

 done we have still to determine the effects of increasing 

 intensity of current brought about by the application of 

 increasing e.m.f. 



The Potential Slide 



For the application of a graduated increase of e.m.f. 

 up to 12 volts or thereabouts, a Potential Slide is very 

 convenient. This consists of two platinoid wires stretched 

 side by side. Two spring-contacts e and e' are carried by 

 a slide between the two wires. When a battery of six 

 storage-cells is applied at the terminals of these wires, at 

 A and B, the difference of electric potential at the two points 

 is 12 volts. When the slide is at the extreme end, to the 

 right, the difference of potential between E and e' will be 

 zero, whereas in being moved to the left the difference of 

 potential or e.m.f. between E and e' will continuously increase 

 till at the extreme left it is 12 volts, e being, say, positive, 

 and e' negative. The electrical current enters the plant 



