I.] ENTR Y OF PLANT FOOD 19 



be present in solution in the external soil water, both 

 will continue to diffuse through the cell wall until their 

 respective concentrations are the same within and 

 without the cell. If now the potassium compounds 

 be withdrawn from the solution within the cell by 

 the living protoplasm in order to take part in the 

 various vital processes requiring potassium, there 

 will be a fresh influx of potassium until the old equi- 

 librium within and without is restored. It is in this 

 way that the apparent selective action of a plant 

 takes place ; as a rule, sodium compounds are more 

 abundant in soil water than salts of potassium, yet 

 the ash of the plant will be found much richer in 

 potassium than in sodium. Similarly, again, the ash 

 of any particular plant will maintain a fairly constant 

 composition although grown on soils of widely differing 

 character. The selective power resides in the living 

 cells themselves ; all substances dissolved in the soil 

 water diffuse through the walls of the root hairs into 

 the plant, but will not continue to accumulate therein 

 unless they are utilised and withdrawn from solution 

 by the protoplasm. 



Further, it is not necessary to consider that the 

 plant takes up the various salts presented to it as 

 wholes ; the process of diffusion until equilibrium is 

 attained, of withdrawal by the protoplasm and con- 

 sequent renewal of the process of diffusion, takes 

 place for each acid or base independently of the 

 others. As a rule, a plant growing in a nutrient 

 medium containing nitrates as sources of nitrogen, will 

 withdraw an excess of acid and render the solution 

 alkaline, but cases also occur when the medium 

 becomes acid during growth because the plant takes 

 more base than acid. According to modern views of 

 solution, we must regard the soil water as a highly 



