THE MATERIAL INCOME OF PLANTS 
35 
of solution is ad ; oined by a zero concentration; the gradient is " steep " 
because the solute at infinite pressure adjoins the pure solvent of zero 
pressure. But the rate constantly falls as diffusion progresses, since the 
difference at any two points is becoming less and less. The rate is also 
greatly influenced by temperature, an increase accelerating and a de- 
crease retarding the rate, exactly as in gases. 1 
Osmosis. Returning now to the conception of the relation of water 
to the plant cell : it might seem that, given waterways in cell wall and 
protoplast, any solute, inside the plant or out, might diffuse in any direc- 
tion in which its concentration is lower. And this would be the case 
were there no relation existing between the solutes and the material 
cf the separating membranes, the cell wall and protoplasm. These 
modify the free diffusion; diffusion through membranes or partitions 
is distinguished as> osmosis, and the pressure which solutes may exert 
on the container is known as osmotic pressure. 
Unlike gas pressure, to which it is comparable, osmotic pressure cannot be mea- 
sured directly except with great difficulty. It is calculable from the amount by 
which a solute lowers the freezing point and raises the boiling point of the solvent. 
Permeable and impermeable membranes. Suppose in a closed glass 
vessel (fig. 622) a glass partition divide A, pure water, from B, a watery 
solution of salt. No interchange _ 
of water or salt between A and B 
is possible through such a parti- 
tion, whence it is said to be im- 
permeable. But if the partition 
be made of some substance with 
whose particles salt particles can 
mingle a substance, that is, 
with which salt forms a solid or 
semi-solid solution then the salt 
- A - 
FlG . 622 . - Diagram : A, pure water; 
B, watery solution of salt, or sulfuric acid; 
> P rti n of \ he . partition su PP sed to te 
removable ; a, b, air. 
particles which by diffusion reach 
the A side of the partition may fly off thence into the water, a; and 
they will do so, provided the attraction of the water for the salt is 
greater than that of the partition stuff for the salt. The nature of the 
partition, then, determines whether any substance may pass through it, 
and of course modifies the rate of its diffusion. 
1 To avoid misunderstanding it may be necessary to add that under like conditions 
each solute diffuses at a rate peculiar to itself. 
