310 RESEARCHES ON IRRITABILITY OF PLANTS 



The Refractory Period 



A striking characteristic of the rhythmic cardiac tissue 

 of the animal is its long refractory period. It is found that 

 by applying successive stimuli there is a minimum resting- 

 interval, the diminution of which brings about such a loss 

 of excitability as to abolish response. This minimum 

 interval is known as the Refractory Period, because short 

 of this period the tissue takes no account of stimulus or is 

 refractory to it. In the cardiac muscle the refractory period 

 lasts during the entire period of the contraction. 



In the response of the leaflet of Desmodium also we 

 observe in this respect a very striking similarity, in its 



Fig. 154. — Record demonstrating refractory period in Desmodium. 

 In the lower record the second stimulus, applied after 45 

 seconds, is seen to be ineffective, having fallen within the 

 refractory period. In the upper record the second stimulus, 

 applied after an interval of 90 seconds, is seen to be effective. 



possession of a long refractory period. This will be seen 

 in fig. 154, where the leaflet in a state of standstill was sub- 

 jected to two successive stimuli of induction-shocks. In 

 the lower record of the two, after the responsive movement 

 due to the first stimulus, a second was applied after an 

 interval of 45 seconds. The leaflet, however, took no 

 account of it, there being induced no second response. The 

 refractory period in this case is seen to be longer than 

 45 seconds. After a suitable interval the same leaflet was 

 subjected to two successive stimuli, and at an interval of 

 90 seconds. It is seen from the upper record that the 

 second stimulus was effective, having fallen on the leaflet 

 beyond the refractory limit. 



