POLAR EFFECTS OF ELECTRICAL CURRENT 201 



left, the pulvinus was suddenly made the kathode, and this 

 induced an excitatory fall of the leaf. It has been said 

 that the exciting action is in general induced by sudden 

 variation, and not during continuation of current. Hence, 

 if after the excitatory fall the current is continued, the leaf 

 is found to re-erect itself and to have its excitability restored. 

 The current is now broken ; this induces no excitation. 

 The commutator is then tilted to the right, the pulvinus 

 being made the anode ; this again induces no excitation, 

 nor is there any further excitation at the break of the anode. 



It is thus seen that with a feehle current the polar effects 

 in Mimosa are precisely the same as in animal tissues — 

 namely, excitation only at the make of the kathode. 



Having thus described the fundamental reaction of plant 

 tissues under feeble currents, we have still to study the 

 effects of stronger currents and the modifications that may be 

 induced in the normal reactions under changing conditions, 

 both internal and external. Indisputable evidence on 

 such points can only be obtained if it is possible to have 

 automatic records made by the plant itself. This I have 

 been able to secure by the method of record already described, 

 with the addition of a contrivance by which definite signals 

 are marked at the base of the record, at the moment of 

 application of either kathode or anode, its continuation, 

 break, renewal, and so forth. For this a rotating reversing 

 key has a disc attached to the axis. Semi-rotation of this 

 axis to the left or to the right, or its position half-way 

 between, renders the pulvinus kathode or anode, or suffices 

 to break the current. The disc has a thread wound round 

 its circumference, so that when the commutator-axis is 

 rotated in one direction the thread is wound, and in the 

 opposite direction unwound. This thread is ultimately 

 led to, and wound about, a second wheel with a long index, 

 which is to serve as the marker. This second wheel is 

 suitably fixed above the recording-plate, with the tip of the 

 index resting on it at the same vertical line but a little 

 lower than the response-recorder. When the commutator- 



