6 PLANT PHYSIOLOGY 



tides. At present this term designates the colloidal particle with 

 all the ions retained by it. The magnitude of the electric charge 

 obtained by the colloidal particle from the ions adsorbed on it is 

 not constant but varies with the concentrations of various electro- 

 lytes in the dispersion medium, in the case of hydrosols, or sols in 

 which water is the dispersion medium. If an electrolyte is added, 

 the positive ions of wdiich will be adsorbed on the colloidal parti- 

 cles more than the negative ions, an increase of the concentration 

 of this electrolyte will cause a gradual decrease of the charge of 

 the particles, until finally they will be entirely neutralized. 



This will be at the isoelectric point of a given colloidal solution. 

 The electric charge of the particles causes their reciprocal 



repulsion and determines to a 

 considerable degree the stability 

 of the colloidal solution. At the 

 isoelectric point, when the charge 

 is neutralized, the colloidal parti- 

 cles may be easily precipitated. 

 Fig. 2. — Electric charge on a col- With a further increase of the 

 loidai particle in water, showing the concentration of the electrolyte, 



double layer of + and — charges. \ _ ^ > 



the colloidal particles take on an 

 opposite charge, and at the same time the stability of the 

 colloidal solution increases. 



The dependence of the magnitude and kind of the electric 

 charge of the colloidal particles upon the presence and concen- 

 tration of electrolytes explains the great importance of the 

 latter, in particular of hydrogen (and hydroxyl) ions, for the 

 life of the plant. From the point of view of colloidal chemistry, 

 protoplasm completely deprived of electrolytes could not exist, 

 for the colloids forming it would be extremely unstable. The 

 adsorbing force of colloidal particles for electrolytes is so high 

 that the presence of even very small amounts of electrolytes 

 insures a sufficient stability of colloids. Consequently, electro- 

 lytes form just as indispensable a part of the protoplasm as do 

 organic colloid substances of the type of the proteins and lipoids. 

 The general trend of the vital processes in the organism is to a 

 considerable degree influenced by the complicated interrelations 

 of colloids and electrolytes. 



One of the most important changes that occurs in hydrosols 

 under the influence of electrolytes is their coagulation. In 



