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 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 



