138 VEGETABLE PHYSIOLOGY 
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 from 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 salt by 
bringing its roots into contact with a strong solution 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 concen- 
tration 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 concentration of a solution of any 
salt is not the same for all plants. 
In like manner the salts which different plants absorb 
vary in amount. 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 ig well 
illustrated by the amounts of silica which can be taken up 
by grasses and by leguminous plants respectively. In an 
ordinary pasture there are always found several kinds of 
grasses, together with clover and other allied plants. An 
analysis of these will show that the ash of the grasses may 
