154 PLANT RESPONSE 



erection of the leaf. There is another and very interesting 

 point of view, from which we may see in this phenomenon the 

 continuity of fatigue with death. In the curve of reversal, 

 due to fatigue, in Mimosa^ we saw that the first contraction, 

 induced by strong and long-continued stimulation, passed into 

 subsequent relaxation. In this latter state, the molecular 

 condition was such that responsiveness was abolished. It was 

 as if, in other words, the tissue had passed into a temporary 

 state of death. It is true that, if the stimulation had not 

 been excessive, the organ would recover its sensitiveness, 

 after a period of rest. But this transient would pass imper- 

 ceptibly into the permanent condition of death if, on the 

 other hand, stimulation had been excessive. In that case, 

 after the fatigue-reversal, the tissue would remain permanently 

 irresponsive. 



I have already said that the death-spasm is an instance 

 of excitatory response to intense stimulation, and we should 

 therefore expect the same kind of effect to be produced as is 

 caused by excessive stimulation, that is to say, a preliminary 

 contraction, followed by relaxation, after which there is no 

 recovery. Again, we saw that in the fatigue-reversal of 

 Mimosa, the subsequent erection of the leaf, mainly due as it 

 was to relaxation, was possibly also aided by the later con- 

 traction of the less excitable upper half of the pulvinus. 

 Similarly, it might be expected that the death-contraction of 

 the less excitable upper, would take place slightly later than 

 that of the lower, half of the pulvinus. We have also seen 

 that the excitability of a tissue declines with age, and this 

 decline would naturally be greater in the more excitable half. 

 Thus the difference of excitability as between the two halves 

 would at the same time tend to disappear. As, then, this 

 spasmodic movement in dorsi-ventral organs is a true instance 

 of differential excitatory response, it would appear that the 

 younger the organ, the greater is the excitatory spasm caused 

 by death, and in experimenting with Mimosa I have found 

 that at the death-point hardly any spasmodic movement is 

 shown by old leaves. These considerations will also explain 



