ABSORPTION OF MINERAL ELEMENTS BY PLANTS 247 



an energetic mobilization of the reserve substances takes place, 

 for instance, in germinating seeds. 



When plants or animals decay, the x)hosphoric acid is liberated 

 largely in the form of inorganic salts, which are available to 

 plants. Complex organic compounds of phosphorus can be 

 utilized only with great difficulty. This has been proved by 

 numerous sterile cultures of plants that included such substances. 

 If the cultures were successful, i.e., if they remained sterile to the 

 end, the plants developed very poorly. If by accident they 

 were infected by bacteria that caused a breakdown of the complex 

 phosphorus compounds, then the plants grew much better 

 (Shulov). 



57. General Role of Cations and Their Specific Activity. — The 

 role of the cations in the life of the plant is more complex and 

 less understood than that of the anions of phosphoric and sul- 

 phuric acids, which have been discussed. Usually cations are 

 found in plants in the uncombined condition. Nevertheless it 

 has been estabhshed by artificial cultures that a certain combi- 

 nation of cations is quite necessary to the normal development 

 of a plant and that none of them, neither potassium, magnesium, 

 calcium, nor iron, can be excluded from the nutrient solution or 

 replaced by another element. Thus each of them must have 

 oome specific function. 



The great advances that are being made in the study of 

 colloidal chemistry begin to reveal the physiological significance 

 of the cations, though this subject is still far from being com- 

 pletely known. As has already been seen, 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. 



Of the forces that hold the colloidal particles suspended in the 

 Uquid, preventing their precipitation and their adherence to 

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

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

 suspending hquid is of the opposite charge. Hence their mutual 

 repulsion. When an electrolyte is introduced into a colloidal 

 solution, the electrostatic equilibrium is immediately changed. 

 There results either a diminished or an increased charge of the 

 particles, which causes their precipitation, or an increased sta- 

 bility of the colloidal suspension (see Art. 2). 



