Translocation in Plant Tissues . 
295 
move, tend to accumulate at that surface, while those increasing surface 
energy will recede from the surface. 
It is therefore probable that the protoplasm shows a more or less 
stratified structure, in accordance with the surface energy relations of the 
chemical compounds present. Lipoids, for example, owing to their great 
power of lowering surface tension, would tend to dispose themselves in the 
outermost layers of the protoplasm. 
It is important to bear in mind that in such a complex colloidal system 
as is here contemplated, since the surface of the components is highly 
developed, adsorption phenomena will occur to a very large extent. 
Substances in solution inside the vacuole may therefore become con- 
centrated at every surface of separation between two different phases, in 
such a way as to secure the maximum lowering of the energy of the system 
as a whole. 
The protoplasm may then be pictured not merely as a combination 
of a suspension of particles and an emulsion of droplets in an aqueous 
medium, with probably also other components in the form of gels, but in 
addition account must be taken of the fact that in the watery phase various 
solutes are present, and furthermore that they may exist there in a con- 
centration differing from that obtaining at the actual surface of separation 
of the water from the other phases. 
It is well known that a reversible equilibrium exists between the con- 
centration of a solute in its solvent and at the surface of an -adsorbing 
substance introduced into the solution. 
To take an example , 1 if equal amounts of charcoal are shaken up with 
dilute acetic acid in known concentrations, a certain proportion of the acid 
is removed from each solution owing to local concentration, or adsorption, at 
the surface of the charcoal particles. 
Quantitative estimations have shown that from the weaker solutions 
a relatively larger proportion is removed than from the stronger ones. This 
relation can be expressed by a curve of the type shown in Fig. 1. 
Put in another way, if to a very fine suspension of charcoal in water 
acetic acid is added, and the mixture well shaken, the acid will distribute 
itself throughout the system in a definite manner such that the concentration 
at the surface of the charcoal bears a specific relation to that of the acid 
remaining in solution. 
If more acid is then added, a new state of equilibrium will be obtained, 
the concentration of acid becoming increased at the charcoal surface, but 
relatively more increased in the solution. 
Similarly, if more water is added to the original solution, the con- 
centration of the acid at the surface of the charcoal will become decreased, 
but that of the acid in solution will become relatively more decreased. 
1 Philip ( 1918 ), pp. 227-30 and p. 232. 
