UNIVERSALITY OF SENSITIVENKSS IN PLANTS 



37 



and also the normal negative electrical response. We have 

 already seen, in fig. 17, an instance of this abnormal positive 

 followed by the normal negative mechanical response, in 

 experiments with BiopJiytuin. This abnormal positive re- 

 sponse, being a matter of the intensity of the blow delivered 

 by the water-movement, can only exhibit itself under favour- 

 able conditions. It is thus possible in mechanical response to 

 have either the normal response preceded by the abnormal, or 

 the normal response alone. But whenever we have an abnormal 

 mechanical response, due 

 to positive turgidity- 

 variation, we have also 

 simultaneously an ab- 

 normal positive electrical 

 response. In fig. 27 is 

 shown a simultaneous re- 

 cord of the two, in which 

 there is a preliminary ab- 

 normal positive mechani- 

 cal response, and a syn- 

 chronous positive elec- 

 trical response, followed 

 in both cases by the 

 normal responses.' It 

 should be stated here that 

 this positive turgidity- 

 variation, which is re- 

 ferred to as abnormal, is 



of very great importance, and will be seen in Chapter XXX. 

 to be directly responsible for growth. It will be found 



Fig. 27. The Abnormal Positive preceding 

 tiie Normal Negative in Mechanical and 

 Electrical Responses in Biophytiim 

 represents the moment of application of 

 stimulus. The upper is the mechanical 

 and the lower the electrical record. The 

 records downward indicate erection of the 

 leaf or galvanometric positivity. 



' It must be understood that this positive electrical response, being dependent 

 on the excitatory expulsion of water at a distant point, is, in a certain sense, a 

 physiological response. For it is the excitability of that distant point which 

 determines the positive turgidity and concomitant positive electrical variation of 

 the point under examination. The contraction of the excited point gives rise to 

 a hydrostatic disturbance, by which a movement of water is brought about. 

 Such a disturbance, then, will be indifferently designated as the hydrostatic, or 

 hydraulic, wave. 



