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



write all results in terms o£ the electromotive force of a standard cell, 

 o£ which the letter d is understood to denote the hundredth part. 



When the whole of the outer surface of the leaf is covered with a 

 mass of clay moistened with salt-solution, and the mass is brought into 

 contact with the movable electrode, the fixed contact being, as before, 

 with pot or petiole, the effect of mechanical excitation is to produce a 

 negative excursion, indicating a change of potential at the movable 

 contact of from 3-5c? to 5*0c? : when a similar plug is applied to the in- 

 ternal surface, so as to cover the whole of it, the result is the same, but 

 the extent of the excursion is somewhat less. Hence it may be generally 

 stated that the electrical disturbance (which, as will be afterwards shown, 

 lasts for about a second) consists in this, that the surface of the leaf 

 becomes more negative as compared with any other surface of which the 

 potential is constant, and that, on the external surface, the change is 

 greater than on the internal. 



The electrical disturbance is strictly limited to the surface of the leaf. 

 If, the fixed electrode being in contact with the petiole, the movable 

 one is brought into contact first with the midrib at its middle and then 

 in succession with points nearer and nearer to the petiole until at last 

 the line is crossed which divides the petiole from the isthmus or bridge 

 by which it is united with the leaf, and the leaf is excited after each 

 change of contact, it is found that each excitation is followed by a 

 negative excursion so long as the contact is on the leaf side of the liyie re- 

 ferred to, but that as soon as that line is passed no sign of electrical dis- 

 turbance manifests itself. Hence if two points of contact, a and 5, are 

 selected on opposite sides of the limiting line, and the movable electrode 

 so shifted alternately from one to the other, an excursion of Id or 2cl is 

 observed in the one case, while in the other the mercurial column re- 

 mains absolutely motionless. "We shall hereafter see that the fact that 

 the bridge or isthmus is electromotive is of importance to the under- 

 standing of certain phenomena. 



Section 4. — On the relative Intensity of the Excitatory Electrical 

 Disturbance at different parts of the Surface of the Leaf. 

 We have already seen that the area of disturbance does not exceed 

 that of the leaf itself. In our endeavour to limit that area further, or 

 at all events to determine the position of the centre of greatest intemity, 

 we have allowed ourselves to be guided by the consideration that as the 

 disturbance itself must be regarded as correlative with the property pos- 

 sessed by the leaf of contracting when excited, that centre is likely to 

 have its seat in the excito-contractile part of the organ. Now there can 

 belittle doubt that the excito-contractile property is localized in that 

 part of the internal surface on which the sensitive hairs are planted, and 

 further that the tissue which takes an active part in the changes o£ form 

 by which that property manifests itself is the parenchyma, of which an 



