200 RESEARCHES ON IRRITABILITY OF PLANTS 



made at or near the pulvinus, and the other, with a distant 

 indifferent point, lower down on the main stem. 



For suddenly starting or stopping a current, or for making 

 an electrode anodic or kathodic, we may use Pohl's Com- 

 mutator or Reverser (fig. 106). When the commutator is 

 in the middle position, the current is interrupted ; when 

 tilted to the left, the current enters the plant through the 

 stem and leaves it by the pulvinus, which thus becomes the 

 kathode. On partially tilting the commutator to the right, 

 the current is broken. On tilting further to the right, the 



Fig. 106. — Pohl's commutator, for causing make and break of 

 kathode or anode. 



current is suddenly reversed and the pulvinus becomes the 

 anode. 



For experiment I took a leaf of Mimosa which was in 

 a fairly sensitive condition ; the electrical connections were 

 made in the manner already described. The electrical 

 resistance offered by the plant tissue between the two points 

 of contact was found to be half a million ohms. An e.m.f. 

 of 4 volts was found effective in inducing excitation at 

 make, when the pulvinus was the kathode. The intensity 

 of the exciting current was feeble, being 8 micro-amperes. 

 A micro-ampere, it should be noted, is one-millionth part 

 of an ampere. When the commutator was turned to the 



