72 TEXTBOOK OF PLANT PHYSIOLOGY 



and less understood than that of the anions of phosphoric and 

 sulphuric acids which have been discussed. Usually cations 

 are found in plants in a free state. It has been established by 

 artificial cultures that a certain combination of cations is quite 

 indispensable for the normal development of a plant and that none 

 of them, neither potassium, magnesium, calcium, nor iron can 

 be excluded from the nutrient solution nor replaced by another 

 element. 



The great advances which are being made in the study of col- 

 loidal chemistry begin to reveal the physiological significance of 

 the cations, though we are still far from knowing this subject 

 completely. As is shown in the next chapter, the colloidal state 

 represents a condition of unstable equilibrium. Colloids may be 

 regarded as heterogenous systems of fine suspensions of solids in 

 water or of emulsions of small droplets of oil. It must be under- 

 stood that the solid or liquid colloidal particles are much smaller 

 than the particles of the solid or liquid phases of relatively coarse 

 mechanical mixtures. 



Of the forces that hold the colloidal particles suspended in 

 the liquid, preventing their precipitation and their adherence to 

 each other, the electric charge of the particles is of the first sig- 

 nificance. All of them carry the same electric charge, while 

 the suspending liquid is of the opposite charge. Hence their 

 mutual repulsion. When an electrolyte is introduced into a 

 colloidal solution, the electrostatic equilibrium is immediately 

 changed, resulting either in a diminished or an increased charge 

 of the particles and thus causing their precipitation, or an 

 increased stability of the colloidal suspension. 



All vital processes of the cell, such as the movement of proto- 

 plasm, its increase in mass during nutrition, the growth of the 

 cell, etc., are dependent on changes in the stability of the colloidal 

 system. Cell colloids may change from sols to gels and vice 

 versa, as for instance during the growth of cell walls or the forma- 

 tion of starch grains or when chromosomes divide. Thus the 

 electrolytes may be considered as most important regulators of 

 many functions of the cell, and the rate and even the direction 

 of various processes of the cell may be influenced by the com- 

 position and concentration of the salts. 



Since the effect of the ions on the colloids depends on their 

 electric charge and on their specific properties, one should not be 



