74 PLANT RESPONSE 



in wooden or ebonite forms, which are made in halves and 

 hinged, so that one half of the cylinder may be swung back 

 in order to afford easy access to the specimen, for the purpose 

 of adjustment. When closed, the wire is in complete circuit 

 with the electrodes outside. By sending through this wire 

 a strong current of short duration, the sudden rise of tem- 

 perature generated in the chamber causes the stimulation of 

 the tissue. This heat is quickly dissipated, again, by the 

 stream of air charged with vapour, which is in constant 

 circulation. The effectiveness of stimulation will depend on 

 the range, and also on the suddenness, of the temperature- 

 variation. The required thermal stimulus is thus most easily 

 effected by electrical means, the degree of rise of temperature 

 being determined by the strength of the current acting 

 on the heating circuit, and the requisite suddenness of 

 variation being the result of temporary completions of the 

 circuit of definite and short duration. This mode of stimu- 

 lation, I shall, for convenience, designate as stimulation by 

 thermal shocks. 



It is very difficult, using only the hand, to attain the 

 necessary precision in making these brief and equal com- 

 pletions of the circuit several times in succession. Hence, 

 the stimulus not being strictly uniform, the responses are 

 apt to become unequal. I have overcome this difficulty, 

 however, by the construction of a closing key regulated by 

 clock-work, which enables successive stimuli, of equal in- 

 tensity and duration, to be applied automatically, at pre- 

 determined intervals. This is accomplished by means of a 

 clockwork arrangement, which enables uniform and successive 

 electrical or thermal shocks to be applied, while at the 

 same time the intervening periods between successive shocks 

 may be so adjusted as to allow for complete recovery. 

 A radial arm, carried on the axle-rod of the clock, at each 

 complete revolution strikes against a balanced key, which 

 completes the electric circuit. The intervals of successive 

 stimulation may be determined by regulating the speed 

 of the clock. This may be done by suitably inclining the 



