STANDARDISATION OF STIMULUS 47 



other. Hence lateral movement, dependent on differential 

 contraction, cannot take place. But if we take a hollow 

 tubular organ of some ordinary plant, say the peduncle of 

 daffodil, it is clear that the protected inner side of the tube 

 must be the more excitable. When this is cut in the form 

 of a spiral strip and excited by means of an electric shock, 

 we observe a responsive movement to take place by curling, 

 due to the greater contraction of the inside of the strip. 

 This mechanical response is at its maximum at that season 

 which is optimum for the plant. When the plant is killed 

 its response disappears. 



It will be seen that the division of plants into sensitive 

 and insensitive is without any justification. Moreover, by 

 adopting the electric mode of investigation, I have shown 

 that every plant and every organ of the plant is sensitive 

 and responds to stimulus by a definite electric variation. 



We have hitherto referred but vaguely to the question 

 of the intensity of the induction-shock employed as stimulus 

 to induce response. We have observed that on making 

 and breaking a current in the primary coil, instantaneous 

 currents are induced in the secondary. The intensity of 

 the induction-current employed for giving a shock depends 

 in the first place on the intensity of the primary current ; 

 secondly, on the suddenness with which the primary current 

 is made or broken ; and lastly, on the relative distance 

 separating the secondary from the primary coil. The in- 

 tensity of the current can be maintained uniform if we 

 always employ the same battery, say a 4-volt accumulator 

 or storage-cell. As the break of a current is accomplished 

 more quickly than make, the break-shock, as we have seen, is 

 more intense than the make-shock. The plant may, there- 

 fore, be excited by a single make-shock, or by a single 

 break-shock or a double make-and-break shock, or by a 

 sequence of make-and-break shocks, of definite duration, 

 according to the particular requirements of the experiment. 



The intensity of the shock moreover may, as already 



