106 PLANT RESPONSE 



Fatigue being principally due to residual strain, it is to 

 be expected that, other things being equal, strain will be 

 more persistent with stronger stimulus, as has been shown in 

 the last chapter. It was there shown also that the period 

 required for recovery from a strong was more protracted 

 than from a moderate stimulus. From this it follows that 

 the stimulation-frequency which will exactly allow for com- 

 plete recovery, and so give rise to uniform responses, in the 

 case of a moderate stimulus, will not be sufficient for stronger 

 stimulus. Hence, keeping the intervals constant, we may 



FlG. 55. Alternate Fatigue (a) in Electrical Responses of Petiole of 

 Cauliflower ; (b) in Multiple Electric Responses of Peduncle of Bio- 

 phytum ; (/) in Multiple Mechanical Responses of Leaflet of Bio- 

 phytum ; and [d] in Autonomous Responses of Desmodiiun 



obtain uniform responses to moderate, and diminished or 

 fatigue-responses to strong stimulus. 



There is another curious phenomenon, of alternate or 

 periodic fatigue, which I have often observed in the response 

 of plants. The simplest type of such periodic fatigue is that 

 in which responses wax and wane, in regular alternation. 

 Such alternate fatigue is sometimes seen in the electrical 

 response of plants, the multiple response of Biophytum, and 

 also in the autonomous response of Desmodium (fig. 55). 

 Curiously enough, I have sometimes obtained similar alter- 

 nate responses with the compound strip of ebonite and india- 

 rubber, previously described. There are other cases of 

 response in plants, where the variations are cyclic in type, that 



