48 RESEARCHES ON IRRITABILITY OF PLANTS 



shown, be increased by sliding the secondary nearer and 

 nearer to the primary coil. At a great distance the intensity 

 of the shock is very feeble, whereas in the nearest position 

 it is most intense. If a scale be placed to mark the relative 

 position of the secondary to the primary, we may be assured 

 of obtaining an identical intensity of shock whenever we place 

 the secondary at the same point on the scale ; or we can 

 obtain an increasing intensity of stimulus by progressive 

 movement along the scale towards the primary. There 

 is, however, no simple relation between the distance and the 

 intensity — that is to say, equal decrement of distance does 

 not mean equal increment of intensity. All that we are 

 sure of, is that the sliding in of the secondary coil secures an 

 increasing intensity of stimulation. In order to be certain 

 of obtaining quantitative values of intensity, the scale has to 

 be specially calibrated. 



Standardisation of Stimulus 



In subjecting the plant to the secondary shock, if we 

 begin with feeble intensity of stimulus, by placing the 

 secondary at a great distance, and gradually increase the 

 intensity by shding the secondary nearer and nearer, we 

 shall obtain that scale-reading at which the stimulus begins 

 to be effective. This particular intensity, the feeblest that 

 is effective, we designate the minimal stimulus. As we now 

 proceed to increase the stimulation by pushing the secondary 

 nearer to the primary, we find the amplitude of the response 

 is progressively enhanced, and ultimately we reach an 

 intensity beyond which there is no further increment of 

 amplitude. This intensity we designate the maximal stimulus. 

 When the plant is in an exceedingly vigorous condition, the 

 minimal intensity is low and the range between maximal and 

 minimal is narrow. But if the plant be in a less favourable 

 tonic condition, then the minimal stimulus is relatively high 

 and the range between minimal and maximal is wider. 



As different induction-coils have different constants, the 



