232 COMPARATIVE ELECTRO-PHYSIOLOGY 



' In the leaf, observed facts show most conclusively 

 that the two sets of phenomena those of the excited 

 and those of the unexcited state are linked together 

 by indissoluble bands : that every change in the state 

 of the leaf when at rest conditionates a corresponding 

 change in the way in which it responds to stimulation, 

 the correspondence consisting in this, that the sign, that 

 is the direction, of the response is opposed to that of the 

 previous state, so that, as the latter changes sign in the 

 direction from t to 4, , the former changes from | to t ' 1 



In making this statement, Burdon Sanderson was prob- 

 ably guided by the prevalent opinion that response takes 

 place by a negative variation of the existing current of rest. 

 We have seen, however, that this supposition is in fact 

 highly misleading. For, owing to such fluctuating factors as 

 age, season, previous history, or excitation due to prepara- 

 tion, the so-called current of rest may and frequently does 

 undergo reversal. Thus a single excitatory effect might, as we 

 have seen (pp. 175-177) under different circumstances, appear 

 either as a positive or a negative variation of the existing 

 current. The assumption of the universality of response by 

 negative variation is thus seen to be unjustifiable. 



Indeed, it would appear from the description of some of 

 the experiments actually related by Burdon Sanderson him- 

 self, that response did not, even in these cases, always take 

 place by negative variation of the existing current. For 

 instance, while in the leaf of Dioncza in its ' prime ' (upper 

 surface positive) the response is negative, and while this 

 latter becomes reversed to positive, as he tells us, in conse- 

 quence of ' modification ' due to previous excitation (fig. 152), 

 yet he admits that even in these circumstances the upper surface 

 had first returned to positivity (ibid. p. 447). Thus, though 

 the responses of the leaf in its ' prime/ and of the ' modified ' 

 leaf are opposed, yet the antecedent electrical condition of 

 the modified leaf has not in this case undergone reversal. 



PhiL Trans. 1889, vol. 179, p. 446. 



