712 PLANT RESPONSE 



(g) Electrical response. — We have, lastly, to consider 

 briefly the electrical mode of responsive expression. The 

 excitatory contraction, with negative turgidity-variation, of 

 a vegetable, as of an animal tissue, is accompanied by an 

 electrical variation of galvanometric negativity. The in- 

 direct effect of stimulus with its positive turgidity-variation, 

 moreover, has also a concomitant electrical expression of 

 galvanometric positivity (p. 37). 



Different types of response : (a) Uniform response. — 

 These various expressions of response are brought about by 

 the fundamental molecular change induced by stimulus. 

 When stimulus is moderate, and sufficient time allowed for 

 recovery from molecular distortion to the original condition, 

 a series of responses to uniform stimuli will be uniform ; 

 but if very strong stimulus be applied, recovery will only be 

 completed after a long interval. 



(b) Fatigue. — If successive stimuli be applied before 

 complete recovery has taken place, the successive responses 

 will exhibit diminution, or fatigue. Under strong and long- 

 continued stimulation the plant-tissue exhibits, as in the 

 case of tetanised muscle, a fatigue-reversal — that is to say, 

 the contracted tissue passes into what is apparently its 

 original expanded condition ; but the difference between 

 the normal condition and the condition of fatigue-reversal 

 is seen in the fact that, while the former is sensitive to fresh 

 stimulation, the latter is insensitive. The fatigued tissue, 

 however, resumes its original excitability after a period of 

 rest. This fatigue-reversal explains the erection of the 

 Mimosa leaf under continuous stimulation (p. no). We 

 observe similar fatigue-reversals, even in inorganic substances 

 like india-rubber, where the normal contraction under thermal 

 stimulation passes into relaxation under the long-continued 

 action of such stimulation ; and the india-rubber becomes 

 sensitive again only after a sufficient period of rest (p. 120). 

 In connection with this, we sometimes meet with the very 

 curious case of alternate or periodic fatigue, both in living 

 and in inorganic substances. The simplest type of this 



