164 mS Sie, oe d Physiological Botany i 
-may be seen by its colouration, but a very much larger quantity of water 
The process will continue until the two fade 
on each side of the bladder have become ho- 
mogeneous. ‘The cause of the increase of the — 
fluid on one side, and the decrease on the other 
passes the other way. 
FIG. 344. —Apparatus | fer 
measuring osmose; 6 
a vessel filled with cupric 
sulphate closed below 
by a permeable mem- 
brane, and placed in a 
vessel of water. As the 
water passes through the 
bladder to mingle with 
the cupric sulphate the 
level of the fiuid will 
rise in the tube ~ in con- 
nection with the vessel 
6, but will fall at z in 
the outer vessel. 
side of the bladder, is that the water passes 
more easily through it than the solution of 
cupric sulphate. But in other cases a very ~ 
considerable interchange of the two fluids may 
take place without its being perceptible ; zzz. 
when they are of the same colour and pass — 
' through the partition-wall in equal quantities. | 
When two fluids are capable of diffusion— 
which is notalways the case—a greater quantity 
of the less concentrated or lighter fluid usually 
passes through than of the more concentrated 
or denser. Cell-walls act in the plant exactly 
like an animal bladder in the instance adduced. 
In the process just described, the current | 
which passes into an enclosed space such as a 
cell is termed an ezdosmotic, that which passes 
in an opposite direction an exosmotzc current. 
Since the cell-sap is muck richer in 
substances which cause osmose [or is 
of a greater specific gravity] than the 
moisture of the soil which contains in 
solution only a small quantity of salts, 
gases, &c., a considerable endosmotic 
current is set up of the absorbed 
moisture into the cell, while only a 
very small quantity of the cell-sap passes » 
out into the ground. This last pro- 
cess has, however, nothing in common with the elimination 
of products of excretion which takes place in animals ; its 
purpose is rather to render soluble the insoluble constituents — 
of the soil, and thus to enable the plant to absorb them in 
‘the water which it takes up. The membranes of the apices 
of root-hairs of plants which are still in active growth are so aM 
delicate that, in obedience to the force of gravitation, they : . 
