130 VEGETABLE PHYSIOLOGY 



or the water with which they are in contact ; indeed 

 the composition of the soil in which a plant grows deter- 

 mines 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 capable 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 behavour 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 complex 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 equili- 

 brium 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 examina- 

 tion is withdrawn from the sap in any part of the plant, 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 question, 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 ditfi- 



