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 is 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 



