114 PLANT RESPONSE 



reduced, to two minutes ; and it will be noticed that, owing 

 to increasing incompleteness of recovery, the responses 

 were here reduced to the merest indications of twitches. 

 When the intervening periods were still further shortened 

 to less than one minute, the stimuli fell within the refractory 

 period of the tissue, and produced no indication whatsoever 

 of their effect. As an extreme instance, we have the effect 

 of continuous stimulation, already described on p. 109. To 

 be precise, we must remember that in this gradual abolition 

 of response, under quickening stimulation, we not only see 

 the action of diminished excitability, but also of diminished 

 conductivity. 



The erection of the leaf of Mimosa, by the relaxing action 

 of fatigue, may also be assisted by the later contraction of 

 the upper half of the pulvinus. For we have seen that in an 

 anisotropic organ, the less excitable half responds subsequently 

 to the more excitable. The contraction of the upper half 

 of the pulvinus in Mimosa would produce erection of the 

 leaf, and that this might sometimes happen appears probable 

 from the fact that in the erection of the leaf under continuous 

 stimulation it is occasionally found to be lifted above its 

 normal position. 



Summary 



Stimulus, by causing molecular derangement, brings about 

 mechanical response ; and by molecular transmission of 

 disturbance from point to point, the excitation is conducted 

 to a distance. Excitatory mechanical response and conduction 

 of excitation are different expressions of the effect of stimulus 



After a period of rest from the action of stimulus, there 

 is a restoration of molecular equilibrium. The protoplasmic 

 properties of excitability and conductivity are then completely 

 restored. Under such normal conditions responses are uniform. 



A tissue in a sluggish condition has its molecular mobility 

 increased by the action of successive stimuli. This produces 

 the ' staircase ' effect, of gradually enhanced responses. 



When sufficient time is not allowed, there is a residual 



