THE ROOT 153 
soil water from the films surrounding them soaks through their 
cell walls into the root-hairs. This soil water is a solution of 
various mineral salts. In order to understand how this solution 
passes into the root-hairs, it is essential to first understand the 
processes of diffusion and osmosis. 
Dirrusion.—This is the process whereby the particles of sub- 
stances making up a solution tend to be uniformly intermingled. 
It may be observed by introducing a ‘ 
crystal of copper sulfate into a tumbler : 
of water. The crystal slowly dissolves 
and the particles of which it is com- 
posed in time diffuse themselves 
equally throughout the water, so that 
the water is colored uniformly blue. 
The movement of the particles of the 
dissolved copper sulfate from the region 
of greater density or concentration to 
that of less implies a pressure which is 
termed diffusion tension. 
Osmosis.—If the denser copper sul- 
fate solution in the bottom of the tum- 
bler were separated from the less dense 
water above by a semipermeable mem- 
brane such as parchment or bladder, 
diffusion would take place through the oe cede ote 
membrane and the water above would .o.. rapid inflow of liquid 
become colored as proof of this diffu- from A to B has elevated the 
sion. Moreover, the water above the liquid surface within from 64 to 
membrane would pass through it in the Re rad ha och apiatcl amtiaad 
opposite direction and more rapidly 
than the solution of the copper salt. It would continue to do 
so until the solution was of the same density on both sides of the 
membrane. Diffusion through a porous membrane is known as osmosts. 
It is a well-known law of physics that when two liquids or gases of 
different densities are separated by a porous (osmotic) membrane, 
diffusion through the membrane will take place until the density of 
the fluids or gases becomes the same on each side. The diffusion 
will be more rapid from the less dense to the more dense region. 
