PHYSICOCHEMICAL ORGANIZATION OF THE PLANT 5 



covered with a bladder membrane or parchment paper is filled 

 with the liquid to be dialyzed and is then immersed in another 

 vessel in which the water is continually renewed. The crystal- 

 loids diffuse through the bladder and are carried away by the 

 water current. The colloidal solution remains in the dialyzer 



(Fig. 1). 



Colloidal particles distributed in the dispersion medium usually 

 combine with the molecules of the medium. This phenomenon 

 is termed solvation or if the dispersion medium is water, which 

 is the case with plant cells, hydration. A similar connection of 

 the particles of a substance with the particles of the solvent is 

 likewise assumed by many chemists for true molecularly, or 

 ionically dispersed solutions. The 

 capacity of hydration differs widely 

 in different colloids. Hydrophilic col- 

 loids, the particles of which are firmly 

 linked with a large number of water 

 molecules, are distinguished from Vig. i.— a dialyzer. Soiu- 



hydrophobic colloids, which are either tion a, separated from water 5 

 incapable of hydration or whose hydra- ^^ ^ semipermeable membrane. 



tion is not stable. The colloids forming the major part of living 

 substance, such as proteins, show very pronounced hydrophilic 

 properties. 



Like the particles of coarsely dispersed substances, the colloidal 

 particles dispersed in the dispersion medium usually carry an 

 electric charge. The presence of such a charge may be proved 

 by this very simple experiment. Immerse a pair of electrodes 

 in the colloidal solution (sol), and p^^ss an electric current through 

 it. The colloidal particles will congregate around one of the 

 electrodes, while the dispersion medium will accumulate at the 

 other electrode. This phenomenon has been termed '' electro- 

 phoresis." A quantitative study of electrophoresis shows that 

 both phases of a colloidal system are equivalently but oppositely 

 charged. In hydrosols, water is usually charged positively, 

 while the colloidal particles carry a negative charge. 



At the surface of contact of the colloidal particles with the 

 dispersion medium, there arises a double electric layer. This 

 layer is composed of the negative ions adsorbed by the particles 

 and of the positive ions held by them in electrical equilibrium 

 (Fig. 2). Naegeli used the term ^'micelles" for colloidal par- 



