53 
discharge is caused in the heart by the anions in a circulating 
fluid. Tlie long closure of a powerful current allows plenty 
of time for a large number of anions to be liberated at the 
anode, and, what is more important, a large amount of kation- 
proteid to be decomposed at the kathode, so that althouofh 
the excess of anions liberated at the anode may 
not be equal to the threshold number while the 
current is closed, yet, on opening, the sudden rush of kations 
to the former kathode causes a sudden fall in the value of the 
threshold number, for anions, at the anode, so that the num- 
ber of free anions may now be many times the value of the 
threshold number. The fact that it occurs best when the 
threshold number is great (cooled nerves) — when the decom- 
position at the kathode is most marked* — favours this vien-. 
The fact that the ''opening tetanus" is removed by immer- 
sion of the nerve in KiS^O^ shows that it is due to anions, 
since it is removed by an excess of kations. 
15. — The Movements of Plants. 
This theory of the influence of the ion-proteid upon the 
surface tension of protoplasm gives a simple explanation of 
the movements, and especially the heliotropism, of plants. 
It is a well-known fact that, in the presence of chlorophyll, 
green plants, under the influence of light, decompose carbon 
dioxide, retaining the carbon and giving off the 
oxygen — this carbon is built up into carbo-hydrates 
and proteid.f Hence, it is evident that the 
rapidity with which the synthesis of proteid (and 
therefore of ion-proteid) goes on is dependent upon the 
supply of carbon : that is, upon the presence and intensity of 
illumination. Supposing 'a contact difference of potential, dae 
to free ions, exists between the protoplasm of plant cells and 
the cell walls, it is readily seen that at the point where the 
assimilation of free ions into ion-proteid is going on most 
rapidly, this contact difference of potential will be diminished, 
and therefore, as we have repeatedly pointed out, the surface 
tension along the contact surface will be increased. This will 
mean decrease of surface at such points, and comparative in- 
crease of surface at other points : therefore, a cylindrical 
stem, in which assimilation is going on more rapidly on (,ne 
side than on the other, will bend towards the former side. 
But, we have seen that if one side of a o-rowing jDlant 
stem is more strongly illuminated than the other, assimilation 
will be going on more quickly on the illuminated side : there- 
* Tide discussion of eJectrotonus, this paper, section 8, 
t Vide Vine's Physiology of Plants, 1886. pages 140-148. 
