PROPERTIES OE THE LEAP OE DIOHiEA. 
25 
tlie excitable parenchyma of the leaf are associated with the excitatory state, and 
consequently I am led to regard it as possible that these changes may not be inadequate 
to produce secondary changes of the same kind in the petiole- —in other words, that if 
the petiole were led off during the excitatory variation, it would show a variation in 
the same direction. Want of material has rendered it impossible to settle this important 
question experimentally. 
The statement contained in my preliminary note in 1873, that the amputation of the 
petiole always increases and often doubles or trebles the electromotive force of the 
“ leaf current,” in other words, increases the negativity or diminishes the positivity of 
the end of the leaf which is next to it, has been confirmed by further and more exact 
observations. In illustration of it the following experiment may be given. A leaf 
was led off in the way described in my note by the two ends of the under surface of 
the midrib, with the fixed electrode at the distal end, it being still on the plant. The 
difference between the two surfaces was found to be — 0*011 D. On excitation by 
touching a hair a normal variation was observed, consisting of a positive deflection 
followed by a prolonged negative after effect, that is, the same phenomena as were 
described in 1873. On cutting off'the leaf with as long a petiole as possible, no effect 
could be observed on the difference. Half the petiole having been removed without 
effect, the difference being now —0*0112, half the remainder was removed, the result 
of which was that the difference increased (the observation being made as soon as 
compensation could be effected, that is, within a minute after the amputation) to 
— 0*0198. In the course of a few minutes the difference sank to —0*01, that is, a 
little below its original amount. Amputating half of the remainder of the petiole still 
attached raised it at once to —0*023, but it soon sank to —0*017. Clipping off the 
stump of the petiole which still remained, produced only a slight increase to —0*02. 
Throughout these experiments the excitatory variation exhibited the same character. 
We have thus before us two sets of facts relating to the electrical influence of the petiole 
on the leaf, viz. : (1) that a current through the petiole, if directed towards the leaf, 
renders the attached and proximal end negative to the distal, and (2) that removal 
of the petiole produces the same effect. I am unable at present to explain either of 
these phenomena. The nature of the electrical relation between the two organs which 
underlies both must remain undetermined until it is possible to make further 
experiments. 
PART IV. 
The Electrical Effects of Excitation. 
1. The fundamental experiment .—Under this term I propose to designate the 
observation of the electrical effects which present themselves when the leaf is led off 
by opposite surfaces of one lobe, while the other is excited by any of the methods 
MDCCCLXXXII. E 
