PROPERTIES OP THE LEAP OF DIONJEA. 
51 
to something more than mere diminution of resistance. In the leaf the removal of 
resistance is everything. Consequently, so soon as a sufficient number of excitations 
have occurred to accomplish this, it is not only incapable of resuming its original form, 
but of any further response to excitation until, after many hours, the resistance is 
restored. 
If we go a step further and enquire what this resistance consists of, we are led, 
in accordance with the conclusion that has been arrived at by every physiologist who 
has investigated the mechanism of the changes of form of the excitable organs of 
plants, to identify it with turgescence. By turgescence we understand the power 
which living protoplasm possesses of retaining water. In the case of cells which are 
excitable, the immediate effect of excitation is suddenly to diminish this power, and 
thereby to produce a diminution of volume of the cells which is equal to that of the 
water (probably holding diffusible bodies in solution) which is discharged into the 
intercellular spaces. 
Addendum. 
Received November 3, 1881. 
[The following is a Summary of the most important Experimental Results embodied 
in Parts III., IV., and Vi] 
1. When different parts of the surface of the uninjured and unexcited leaf of Dionsea 
are compared by the method of compensation described in Part II., electrical differences 
present themselves even when the points selected are symmetrical. In this case the 
differences observed are accidental. They are due either to surface conditions which 
disappear when the leaf, with its leading off electrodes in position, is allowed to remain 
in saturated air (differences of turgidity of the surface layers of cells) or to accidental 
physiological differences between the two lobes, which do not disappear. 
2. Similarly, when unsymmetrical points of either surface of the leaf are led off, the 
differences observed are for the most part accidental. In one case, however—that of 
the comparison of the middle of the under surface of either lobe with the under surface 
of the petiole—the negativity of the former to the latter is a result which occurs so 
constantly that it must be assumed to have a physiological meaning. 
3. If any two points opposite to each other on the upper and under surface of either 
lobe are compared, it is usually found that the upper surface is negative to the lower. 
Whether this is so or not in the unexcited leaf, it becomes so after one or more excita¬ 
tions ; in other words, the effect of mechanical or electrical excitation, whether applied 
in the neighbourhood of the surfaces compared or at a distance from them, is to pro¬ 
duce a lasting change in their electrical relation of such a nature that on repeating the 
comparison of the two surfaces the under surface is always found to have become more 
positive (less negative ) than it was before excitation. 
H 2 
