136 VEGETABLE PHYSIOLOGY 
of the soil in which a plant grows determines to a very great 
extent what minerals enter it. Ifa particular substance 1s 
soluble in the liquid which the root-hairs absorb, and is cap- 
able 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 be- 
haviour 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 com- 
plex 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 equilibrium 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 If the substance under examination.is withdrawn 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 longas 
such a withdrawal at any point takes place, and a stream 
of the substance will flow continuously to the point in ques- 
tion, so that the process of absorption will be continuous also. 
Some of the materials found in the soil are readily 
soluble m 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- 
culty if they are capable of passing through the protoplasm 
of the root-hair. The solution of the salts is always very 
dilute, and, on account of the ready diffusion that takes 
