H4 RESEARCHES ON IRRITABILITY OF PLANTS 



conducting-strip, so long there will be a deflection in the 

 galvanometer. By means of its screw- adjustment, the rod 

 is gradually raised till the line of junction between platinum 

 and ebonite is exactly reached ; the deflection in the 

 galvanometer will now cease suddenly. In this way the 

 point of interruption or ' break ' is determined with precision. 

 By pulling the thread in connection with one arm of the 

 recording-lever, we then trace a slightly curved line on the 

 smoked plate. This indicates the exact position in succeed- 

 ing records of the moment of application of stimulus. This 

 mark of stimulation is shown in the printed records as a 

 vertical line. After making this mark on the plate, the 

 peg is removed. It is easy to see that in successive experi- 

 ments stimulation will occur at that definite moment which 

 corresponds to this marked line of stimulation. 



k' represents a key-device by which the make-shock 

 is prevented from exciting the plant. One end of a lever 

 carries a bent metal-rod of u-shape, which is partly immersed 

 in cups of mercury by means of a spring. During the 

 depressed position of this key, the secondary coil is short- 

 circuited. When the handle k is slightly pressed, there 

 is a ' make ' of the primary current. But the make-shock 

 is short-circuited as k' is still in the depressed position. 

 Further pressure of the handle K lifts k' up, removing the 

 short-circuit of the secondary coil. When the break-shock 

 is given by the contact-breaker of the falling plate, there 

 is no short-circuit to divert the shock, which now passes 

 through the plant and excites it. 



The sequence of these events, then, is as follows : — 

 By turning the disc d the recording-plate is lifted and 

 held arrested in the up-position. The pressure of the 

 handle k releases the plate-carrier, which then begins to 

 fall. At the same time, the primary circuit is completed 

 and a make-shock is induced in the secondary. But this 

 make-shock is diverted by the short-circuit key k', which 

 is still in the depressed position. Further . pressure of the 

 handle k removes the short-circuit by lifting the ends of k'. 



