130 VEGETABLE PHYSIOLOGY 
or the water with which they are in contact; indeed 
the composition of the soil in which a plant grows deter- 
mines to a very great extent what minerals enter it. If a 
particular substance is soluble in the liquid which the root- 
hairs absorb, and is capable of osmosis through their 
membrane, a certain quantity will, by ordinary physical 
processes, be taken up by them. 
It does not, however, follow that, if the conditions 
alluded to are realised, absorption of a particular salt will 
go on indefinitely. The quantity of any substance which 
a plant will absorb will depend upon whether it is made 
use of in any way, or can be deposited in its tissues in an 
insoluble form. This can be seen most easily by studying 
the behavour of a single cell. If any substance which 
enters the cell by osmosis is used in its metabolism, it will 
be quickly removed from the sap in its vacuole, and more 
will enter. If not, the cell-sap will soon have taken up as 
much of it as it can contain, and the absorption of that 
particular substance will cease. This is equally true of 
such a complex of cells as constitutes a plant, though the 
time of the absorption will be more prolonged. As soon as 
all the cells of the complex attain a condition of equili- 
brium with regard to the particular salt in question, no 
more will be taken up. This follows from the nature of 
the process of osmosis. If the substance under examina- 
tion is withdrawn from the sap in any part of the plant, 
and made use of for any purpose, or deposited in the cells 
in an insoluble form, the condition of equilibrium will not 
be attained so long as such a withdrawal at any point takes 
place, and a stream of the substance will flow continuously 
to the point in question, so that the process of absorption 
will be continuous also. 
Some of the materials found in the soil are readily 
soluble in the water which surrounds its particles. We 
have already seen that it is only this hygroscopic water 
which finds its way into the root-hairs. Such salts dis- 
solve in this water and can enter the plant without diffi- 
