RESPONSE OF LEAVES 231 



is an explosive molecular change, which by the mode of 

 its origin, the suddenness of its incidence, and the 

 rapidity of its propagation, is distinguished from every 

 other phenomenon except the one with which I have 

 identified it namely, the corresponding process in the 

 excitable tissues of animals. Of the nature of this 

 preliminary disturbance (to which alone the term ex- 

 citatory variation ought to be applied, it alone being 

 the analogue of the ' action-current ' of animal physio- 

 logy) we know nothing. . . . The direction of the ex- 

 citatory effect in the fundamental experiment is such as 

 to indicate that in excitation, excited cells become 

 positive to unexcited, whereas in animal tissues excited 

 parts always become negative to unexcited. The ap- 

 parent discrepancy will probably find its explanation in 

 the difference of the structural relations of the electro- 

 motive surfaces.' 1 



From this quotation it will be seen that Burdon 

 Sanderson had fallen into the basic error of mistaking what 

 I have demonstrated to be the hydro-positive, for the true 

 excitatory effect, and vice versa. 



In a subsequent Paper again (Phil. Trans, vol. 179, 1889) 

 Burdon Sanderson published certain results, which differed 

 from those referred to above. He had previously found that 

 usually speaking the upper surface of each lobe was negative 

 to the lower. Later, however, he came to the conclusion 

 that in the leaf of Dion&a in its ' prime,' the upper surface 

 was positive to the under. On repeating his * fundamental 

 experiment' moreover, with these vigorous leaves, he found 

 that instead of the pronounced positive response which he had 

 previously observed, he now obtained a short-lived positive 

 effect succeeded by a strong negative (fig. 151). He was 

 unable to offer any definite explanation of this difference 

 between the two sets of results, but suggested that it might 



Iirise, in some way, from changes of the resting-current. 

 1 Phil. Trans. 1882, vol. 173, p. 55. 



