4 
PROFESSOR B. SANDERSON ON THE ELECTROMOTIVE 
angles to the midrib. Accordingly, the leaf ought, on the assumption that the cells 
have the properties assigned to them (i.e., that the middle of each cell is negative to 
its ends), to be represented by a bilateral schema consisting of two symmetrical parts, 
resembling in form the two lobes of the leaf, and each made up of metallic cylinders 
arranged like the cells in the leaf, as above described. A full account of the properties 
actually demonstrable in such an arrangement is given in the paper (pp. 54 and 55). 
It corresponds in the most important respects, as regards the distribution of electrical 
tension, with Professor Mttnk’s account of the electrical properties of the resting leaf. 
§ 5 relates to the “ Mechanism of the Excitatory Motion.” On this subject it will 
be sufficient to state that the account given by Professor Munk of the way in which 
the leaf closes when touched, agrees with the explanation originally given by Brucke 
of the mechanism of the motion of the sensitive plant; namely, that the immediate 
cause of the change of form is the sudden passing of certain cells (in the case of Dionsea 
those of the upper (inner) layer of the lamina and midrib) from the state of distension 
into that of flaccidity ; but, in addition to this, there is, he says, an active lengthening 
and extension of the cells of the lower or external layer.* From the context it 
appears that all that is here meant is that the water discharged in the sudden 
relaxation of the cells of the inner (upper) stratum, finds its way at once to the outer 
(lower). At all events, the only evidence given in support of active extension is the 
fact, proved by measurement, that two marks on the external surface of the expanded 
leaf, in a line parallel to the veining and at a measured distance from each other, are 
found to be further apart after the leaf has closed (p. 117). 
In § 6 (p. 123) the author approaches the question which more immediately concerns 
our present purpose, viz. : the electrical phenomena which result from excitation. I 
will endeavour to give an account, first of the experiments, and secondly of the theo¬ 
retical explanations—premising that, in this as in the section on the electromotive 
properties of the unexcited leaf, observations and theories are so mixed that it requires 
much patience to separate them. 
The mode of experiment was always the same. The leaf was led off, either by 
different points on the under surface of the midrib, as in my original experiments in 
1873, or by the midrib and by a spot on the external surface opposite to it. The leaf 
was not fixed or restrained, so that only a very few excitations could be observed 
before closure. 
All of the experimental results are stated in the following table, in the first 
column of which I have entered the leading off contacts, distinguishing them by 
the letters A and B, in the second column whatever information is given as to the 
difference of potential between them before excitation. The third column gives the 
character of the excitatory effect as observed by the galvanometer; and the fourth, the 
reference to the page where the experiment is recorded. 
* “ Die Bewegung kommt daduroli zu Stande dass das reizbare Parencliym erscblafft und kiirzer wird, 
das nicht reizbare sich activ verlangert ” (p. 119). 
