UNIFORM, FATIGUE, AND STAIRCASE EFFECTS I I I 



a period of repose. And this return of the leaf to its normal 

 condition is testified by its once more responding to stimulus. 



Fatigue of conductivity and excitability. — It has already 

 been pointed out that the protoplasmic properties of a tissue 

 cannot be restored to their original condition after stimulation, 

 without the intervention of a suitable period for the re- 

 establishment of molecular equilibrium. This fact I shall 

 now demonstrate by additional experiments. 



The restoration of the normal protoplasmic condition in 

 a tissue may be tested by observing the recovery of some of 

 those properties which are capable of measurement. One of 

 these is its conductivity, measured by determining the speed 

 with which excitation travels through the tissue, the method 

 of which determination will be fully described in Chapter XX. 

 Under normal conditions this velocity is constant. If we 

 excite the tissue, and measure the rate at which the excitation 

 travels, and if we then allow a sufficient interval of rest for 

 complete protoplasmic recovery and again determine this 

 velocity, we shall find that the two are the same. But, if the 

 necessary resting interval be not allowed, recovery being 

 incomplete, there will remain a residual strain. The velocity 

 of transmission of excitation will, under these circumstances, 

 be found to be reduced. 



Another protoplasmic property -which is capable of 

 measurement is the excitatory contraction seen in motile 

 organs, measurable by the amplitude of response. We 

 have seen that this amplitude is constant under normal 

 conditions, and when sufficient intervening periods of rest 

 are allowed. But diminution of the intervening resting period 

 produces diminution of the amplitude of response. 



From what has been said it follows that, if the intervening 

 resting periods of a tissue be continuously diminished, there 

 will be a continuously increasing residual strain, and this 

 might be detected by the consequent continuous decrease 

 of conductivity and motile excitability. I have been able 

 to verify this deduction by an experiment on a leaf of 

 Biophytum, the details of which will be found in Chapter XX. 



