00 
seen— and this will counteract the effect of gravity tending to 
make the leaf hang downwards. 
The importance of sudden change in illumination"*^ is due 
to two factors : one the tendency of growth to counteract 
heliotropism if illumination is carried on for some time, and 
the other the tendency the ions from the unilluminated side 
will have to diffuse faster into the illuminated side, as the ions 
there are assimilated, a tendency winch would slic^htly increase 
the P.D. at first lowered by the assimilation. It is evident 
that in normal growing plants these factors of heliotropism, 
growth, gravity, etc., will eventually reach a state of more or 
less settled equilibrium, which will determine the permanent 
form of woody parts. 
That differences of potential, such as we have described, 
do exist in plants is well known. Thus, Biedermannf men- 
tions that Kunkel found the veins of a leaf "positive" to the 
green surface (translating this physiological terminology this 
means that internally to the leaf there was an E.M.F. tend- 
ing to promote a current from the green parts to the veins). 
There can be no doubt, I think, that this is due to the kations 
of the salts, brought up by the transpiration current, diffusing 
more rapidly through the walls of the vessels in the veins 
than the anions. The salts brouofht up are mainly KNO3 
and KCl, in which the kation has a greater velocity than the 
anion. 1 The same explanation applies to the "negativitv" 
of the roots of a seedling towards the cotyledon, and higher 
parts, § for the roots would have a large supply of kations due 
to diffusion from the moisture in the soil which diminishes 
progressively as the transpiration current mounts up the stem 
and the kations are assimilated. 
Hermann II found that cross sections of the stem of a 
plant were ''negative" to normal parts. This is doubtless due 
to decomposition of kation-proteid at the point of injury 
liberating kations. 
Burdon-Sanderson finds that when the leaf of Dionea 
closes, the lower surface becomes '"negative" to the upper .H 
This affords an explanation of its closure, since kations are 
liberated on the under side the surface tension on that side is 
* Darwin : The Movements of Plants, page 457. 
t Biedermann : Eleotro-physioloo;v : Trans, by F. A. Welby, 
vol. ii., page 2. 
t Tide Table of Stimulation Efficiencies, thi=; paper, sec- 
tion 3. 
§ Biedermann : Electro-physiology: Trans, by F. A. Welby, 
vol. ii.. page 5. 
I Ibid., vol. ii.. page 2. 
H Ihid., vol. ii.. page 23. 
