THE APPLICATION OF QUANTITATIVE STIMULUS 37 



These responses, being physiological, vary in intensity 

 with the condition of the specimen. The same plant which 

 gives strong electrical response in spring or summer, may 

 exhibit but feeble responsiveness in autumn or winter. Again, 

 we shall see in a subsequent chapter that any agent which 

 depresses physiological activity will also depress the electri- 

 cal response; and, lastly, when the specimen is killed, the 

 normal response is abolished. 



I shall next describe a second and equally perfect method 

 of stimulation, by means, namely, of thermal shocks. We 

 have seen that a sudden thermal variation acts as an efficient 

 stimulus. I have also shown in my ' Plant Response ' that 

 thermal radiation acts as a stimulating agent, in inducing 

 excitatory contraction. Hence, if a tissue be surrounded by 

 a platinum wire, through which an electrical heating-current 

 can be sent, the enclosed tissue will be subjected to a sudden 

 variation of temperature, and also to the thermal radiation 

 proceeding from the heated wire. Now if in successive 

 experiments the duration and intensity of the current 

 flowing through the wire be maintained constant, the 

 stimuli also will thereby be rendered constant. The thermal 

 stimulator, as already said, surrounds the specimen, but is 

 not in actual contact with it. This is to prevent any injury 

 to the tissue, by scorching. The current is so adjusted as to 

 make the platinum wire red-hot and this heating-current is 

 closed for about half a second at a time. Should larger 

 response be desired, it can be obtained by the summated 

 effect of a number of such shocks, or the thermal stimulator 

 may be put in direct contact with the tissue, if care be 

 taken that the rise of temperature is not so great as to 

 injure it. 



The difficulty of ensuring similarity of duration to each 

 individual shock is overcome by the use of a balanced key 

 actuated by a metronome (fig. 27). A second rod is attached 

 at right angles to the vibrating rod of the metronome, and 

 carries a bent piece of brass in the form of two prongs. 

 During the course of each vibration these prongs dip into 



