4i6 UTILIZATION OF MINERAL SALTS 



2. Influence on the Osmotic Pressure of Plant Cells. — In Chap. VIII it 

 was shown that a portion of the osmotic pressure of the cell sap of any plant 

 cell is due to the dissolved mineral salts which it contains. While in most 

 plant cells the absolute concentration of mineral salts in the plant sap is so 

 low that only a small proportion of the osmotic pressure can be ascribed in 

 their presence, there are some important exceptions to this statement as shown 

 previously. 



3. Influence on Acidity and Buffer Action. — The mineral salts absorbed 

 from the soil often have an influence on the pH of the cell sap and other 

 parts of plant cells, although usually not a very great one, as organic acids 

 and other compounds resulting from the metabolic activities of plants usually 

 exert the predominant influence in determining pH values within cells. As 

 shown in Chap. VI, two of the important buffer systems found in plants — 

 the phosphate and the carbonate systems — have their origin in substances ab- 

 sorbed by the plant from its environment. The phosphate system, however, 

 is the only one found in plants which may properly be classed as a mineral 

 element buffer system. 



4. Influence on the Hydration of Cell Colloids. — The effects of cations 

 upon the hydration of colloidal materials has already been discussed in Chap. 

 IX. Anions also have an effect upon the hydration of colloidal micelles, but 

 in most organic colloids their effect is less marked than that of cations. The 

 cations and anions in plant cells undoubtedly influence the degree of hj'dra- 

 tion of the colloidal micelles in the protoplasm and other parts of plant cells. 



5. Influence on the Permeability of Membranes. — As already discussed 

 in Chap. X the permeability of the cytoplasmic membranes is markedly in- 

 fluenced by the cations and anions in the medium with which they are in 

 contact. 



6. Toxic Effects of Mineral Elements. — Many mineral elements, in their 

 ionic form, have a decided toxic effect upon protoplasm, often resulting in its 

 disorganization and death. This is especially true of salts of the "heavy 

 metals" such as iron, copper, mercury, etc. Even some of the elements most 

 essential for the existence of plants such as magnesium, calcium and potassium 

 may exert at least a slight toxic effect under certain conditions. This is 

 shown by tlie fact that plants grown with their roots immersed in dilute solu- 

 tions of almost any salt will usually die sooner than similar plants with their 

 roots immersed in distilled water. The toxicity of any element varies greatly 

 depending upon its ionic concentration. Further examples of the toxic effect 

 of ions are mentioned later in this chapter. 



7. Antagonistic Effects. — The antagonism between univalent and bivalent 

 cations in effects upon permeability has already been considered in Chap. X. 



