132 VEGETABLE PHYSIOLOGY 
flower-pot, after a certain time of growth of the plant con- 
tained in it, the plate will exhibit a tracing of the course 
of the roots which have come into contact with it, but, 
instead of being in relief as in the former case, it will be 
etched to a certain depth. The solvent influence can thus 
be seen to come from the root itself, and not the water in 
the soil. It will, in fact, be the acid sap which makes its 
way out of the root-hairs. 
Certain constituents of the soil can be absorbed which 
are made available in neither of the ways mentioned. 
Soils contain many constituents which cannot pass through 
the protoplasm, but which, in the presence of water, react 
with one another, producing new compounds which are 
capable of such osmotic entry and which are consequently 
absorbed. 
The solutions taken in are excessively dilute. We 
cannot make a plant take up a greater quantity of any 
salts by bringing its roots into contact with a strong solu- 
tion of it. There is a certain relation necessary between 
the substance and the water, which has been the subject of 
considerable investigation. For every salt there is a 
particular concentration or strength of solution, which if 
presented to the plant will be absorbed unchanged; if the 
solution found by the roots is stronger than this, relatively 
more water than salt will be taken from it; if weaker 
relatively more salt than water. It is seldom, therefore, 
that a solution is absorbed without a certain modification 
of its concentration. Moreover, the optimum concentra- 
tion of a solution of any salt is not the same for all plants. 
In like manner the salts which different plants absorb 
vary inamount. If two species are growing in the same 
soil, side by side, under exactly the same conditions, 
the amounts of the several salts present in the soil which 
are absorbed by the plants of the different species will not 
be the same. In each case the quantity will vary accord- 
ing to the use the plant can make of it. This is well 
illustrated by the amounts of silica which can be taken up 
