32 THE THEORY OF IONS 



water, up to a certain point, accelerates its activity ; 

 beyond this point it is destructive. Desiccation 

 even to a slight degree will destroy the vitality of 

 the protoplasm of all cells in the higher animals or 

 plants ; but this does not hold good for certain low- 

 formed organisms. We also know that certain salts 

 are as essential to protoplasm as the water which 

 acts as their solvent. But their action shows that 

 an excess of any chemical in a molecular form is 

 deleterious to protoplasm ; and their freedom from 

 injury depends upon their presence in the form of a 

 dilute solution. Plants absorb chemical substances 

 through their root-hairs by osmosis, each ingredient 

 in the moisture around the roots tending to become 

 as abundant inside as outside the root-hair. Absorp- 

 tion by osmosis only takes place when the solution 

 outside is richer than the cell-sap in the absorbed 

 material. But in any case the solution must be 

 dilute, because strong solutions would kill the 

 plant. 



The evidence that salts are largely absorbed in 

 the form of ions is increasing. It is also known that 

 the proportion of ions in the moisture surrounding 

 the roots influences the osmotic pressure, and conse- 

 quently the rate of absorption. It has been stated, 

 vide ante, that the osmotic pressure of electrolytes is 

 equal to the proportion of the contained molecules + 

 ions, that the weaker the solution the greater is the 

 ionic concentration, therefore the pressure is higher 

 and the absorption more rapid in consequence. 



The roots of a plant excrete certain acid excretions 

 which render soluble some of the otherwise insoluble 

 mineral constituents of the soil, and thereby produce 



