The Plant as a Metabolic Unit 241 



Accumulation of Mineral Solutes and Plant Buffer Systems 



It is beyond the scope of this contribution to enter into a general 

 discussion of the function of mineral elements in plant growth, 

 but some comment on the mineral nutrition of the plant in re- 

 lation to the buffer system of the cell sap is useful, as illustrating 

 another problem of soil-plant systems. 



There is a fundamental difficulty in the quantitative examina- 

 tion of the bufiFer systems of complex plant tissues in that some 

 method of expressing sap from such tissues must be employed 

 and the resultant fluid is a composite and to some extent altered 

 sap. Nevertheless, important information has been gained. The 

 tendency to maintain specific hydrogen ion concentrations, 

 within a rather narrow range, is very marked in many plant 

 tissues, as can be observed even under the highly imperfect condi- 

 tions of present experimentation. Phosphates and salts of organic 

 acids are of especial importance in most plant buffer systems. 

 The metabolic activities of the cell determine, on the one hand, 

 the accumulation of the mineral components of the buflfer sys- 

 tem, and on the other hand, the transformations of organic sub- 

 stances to provide organic acid radicles. Oxygen and available 

 carbohydrate supply, temperature, and other factors already dis- 

 cussed, are essentially concerned in both aspects of the processes 

 which operate to maintain a suitable buffering system in the sap. 



The predominate anion capable of ready metabolic trans- 

 formation is the nitrate ion. Nitrate and bases enter the plant 

 and nitrate undergoes more or less rapid reduction followed by 

 the synthesis of organic forms of nitrogen. A basic residue is left 

 and this is neutralized by organic acids. 



The nature of the mineral and organic complexes of the soil 

 determines the inherent supplying power of the soil for basic 



