THE MATERIAL INCOME OF PLANTS 305 



of solution is adjoined 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 diflfusion 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.' 



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 

 of 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 1 ' I wi i M 



is possible through such a parti- 1 '\n." M 



tion, whence it is said to be im- R- | — 1 



permeable. But if the partition 1 'a | — p — M 



•^ 1-1 M ^ 1 _ M 



be made of some substance with 1 . | " S 



whose particles salt particles can ^ ^mmMlMAMMMWii^a 



mingle — a substance, that is, Fig. 6... -Diagram : ^, pure water ; 



with which salt forms a solid or B, watery solution of salt, or sulfuric acid; 



semi-solid solution-then the salt ^' P°'^'!°" °^ '^ .P^^'"°" ="PP°^^<^ *° ^^ 



removable ; t/, o, axr. 



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. 



' To avoid misunderstanding it may be necessary to add that under like conditions 

 each solute diffuses at a rate peculiar to itself. 



