RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 67 
is somewhat modified by a semipermeable membrane between them 
which, in fact, tends to make the diffusion one sided. Expressed 
briefly and without attempting to define the kinetic forces, if there 
is water on one side of the semipermeable membrane and on the other 
side a solute in water, there is a constant tendency of the molecules 
of water to move both ways through the membrane, while the mole- 
cules of the solute, being heavier and larger, do not so readily diffuse 
to the side, where they are deficient in numbers. On the other hand 
the presence of the solute on one side of the membrane evidently has 
the effect of suppressing the energy of the water molecules there, so 
that fewer molecules pass in the outward direction than are coming 
in through the membrane. This results in an accumulation on one 
side of the membrane, and, so far as the increasing volume here is 
restrained, gives rise to a pressure which is termed "osmotic pres- 
sure." Actually, the pressure must be due to the energy of the free 
molecules which bombard the membrane from the outside. 
In the case of the plant, there are, in addition to the mineral salts 
and organic compounds which may be in solution within the root- 
cells, and within each succeeding cell in greater concentration from 
root to leaf tip, two additional factors or forces, which undoubtedly 
have much to do with osmosis. While the forces may be called capil- 
larity and adsorption, respectively, there is no reason for supposing 
that the action of these forces, so far as water molecules are con- 
cerned, is different from that of molecules of solids in solution. In 
other words, a molecule of a solid, whether acting individually as part 
of a relatively solid mass or surface or as part of a gelatinous mass 
like the cell protoplasm, by its gravitational attraction for a water 
molecule tends to suppress the activity of that molecule and thereby, 
under the proper conditions, gives rise to the process we call osmosis. 
The so-called capillary attraction of the cell walls, which cause 
them to imbibe enough water to fill their intercellular spaces, can 
not be considered an important factor in osmosis, for ordinarily this 
attraction would pull the water as strongly from one side of the 
wall as the other. In the case of dead wood cells, it may be imagined 
that a small amount of water is transferred from one point to another 
through no other action than this capillary affinity of the cell walls. 
The really important element in osmosis is, without question, the 
affinity of the protoplasm for water. While it appears to be true that 
a cell similar to a plant cell might have the same water content, aris- 
ing from osmotic pressure and due entirely to inorganic solutes, still 
the important thing is. not merely that the protoplasm must have a 
certain minimum amount of water to maintain its properties as a 
colloid, but that through selective absorption it is able to regulate, at 
least to some extent, the nature of the solution which shall occupy the 
space Avithin the cell. The protoplasm, while directly effective in 
