136 VEGETABLE PHYSIOLOGY 



of the soil in which a plant grows determines to a very great 

 extent what minerals enter it. If a particular substance is 

 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 If the substance under examination is withdrawn 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 long as 

 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 in 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 



