426 Prof. Burdon-Sanderson and Mr. F. J. M. Page [Dec. 14, 



is made. By means of the petiole, the whole plant and the electrode 

 which is applied to it and the capillary tube are in contact with an electro- 

 motive part of the leaf, namely, the " bridge " already mentioned. The 

 bridge, as we have seen, is within the area of electrical disturbance, and 

 participates in the general negative variation ; but by reason of its position, 

 i. e. of the fact that it is virtually in contact with the fixed electrode, and 

 through it with the capillary mercurial surface, its action on the electro- 

 meter, is opposed to that of the surface to which the movable electrode is 

 applied. Supposing the variation at the two contacts to be simultaneous 

 and to have the same sign, they must counteract each other. In general 

 the variation at the surface of the leaf far exceeds that at the bridge, so 

 that the sign of the excursion is purely negative ; but whenever the dis- 

 turbance in the neighbourhood of the bridge happens to exceed that 

 which occurs at the movable contact in intensity, the excursion becomes 

 partially, or entirely, positive. That this is so may be easily shown by 

 the following experiment : — If in a leaf of which the internal lobe-varia- 

 tion is negative, the fixed contact is shifted from the petiole to the end 

 of the midrib next the bridge, it is observed either that the excursion 

 diminishes or becomes at once positive, or that there is a short negative 

 excursion followed by a longer positive one, this signifying that although 

 at first the lobe-variation has the better, it jdelds immediately after to 

 the more intense disturbance at the midrib. 



The principle thus illustrated applies not merely to the particular case 

 now under consideration, but to every case in which the difference of 

 potential is measured between any part of the surface of the leaf and any 

 part of the surface of the plant assumed not to be electromotive. In 

 every such case it must be assumed that the difference measured represents 

 not .the whole of the electromotive force exerted at the surface of the 

 leaf, but only so much of it as is in excess of the electromotive force 

 exerted at the end of the midrib next the bridge. 



"We propose to apply the term " interference excursion " to the effect 

 observed whenever, as in the experiment referred to at the close of the 

 last Section, contact is made T^-ith both electrodes ivithin the area of dis- 

 turbance ; so that the electrical state of the surface is compared, not with 

 another surface of which the potential is constant, but with a surface 

 which is itself a seat of change. 



The characters of such excursions are much more complicated than 

 those we have hitherto considered — a fact which can be readily under- 

 stood when it is borne in mind that, as will be shown in a future section, 

 the electrical disturbance, although it exhibits the same general character 

 everywhere, does not either begin or attain its maximum at the same 

 moment in different parts of its area. To a great extent the characters 

 of interference excursions may be understood, if along with the want of 

 synchronism the differences of intensity of the changes going on at the 

 two contacts during the period of disturbance be taken into account ; 



