8 PLANT PHYSIOLOGY 



The conduct of colloids, when these are being acted upon by- 

 electrolytes and other agents, diverges appreciably from the 

 regularities stated above. This is revealed chiefly in the study 

 of hydrophobic colloids. It is obvious, therefore, that the main 

 factor determining stabilization of hydrophobic colloids is the 

 electric charge, while the main factor determining the stability 

 of hydrophilic colloids is the capacity of the dispersed phase to 

 adsorb various amounts of the solvent. Owing to the stabilizing 

 action of the water layers surrounding each micelle, hydrophihc 

 colloids may retain the properties of sols even at the isolectric 

 point and precipitate (coagulate) only when acted upon by 

 substances that extract water vigorously, e.g., alcohol or acetone, 

 and that are widely applied in biochemistry. This withdrawal 

 of water explains the phenomenon of salting out of proteins and 

 other hydrophilic colloids by concentrated solutions of neutral 

 salts. 



In most cases, coagulation of hydrophilic colloids is reversible. 

 With the addition of water, the precipitate dissolves, form- 

 ing a hydrosol. This distinguishes sharply hydrophilic from 

 hydrophobic colloids; the coagulation of the latter is usually 

 irreversible. 



To understand the phenomena taking place in cells, it is of 

 special interest to study the original coagulation of many hydro- 

 philic colloids, which has been designated by the term of ''gelatin- 

 ization." Jellies have a very fine, complicated structure. 

 Judging from some of their properties, it is to be assumed that 

 the particles of the dispersed phase are continuous. Jellies are 

 supposed to represent a fine network of colloid micelles, the 

 interspaces of which are filled with molecules of the dispersion 

 medium. This network structure accords well with the elasticity 

 of jeUies as well as with their property of sweUing. 



The process of swelling determines the capacity of jelUes to 

 absorb water or other corresponding solvents and thereby to 

 increase considerably in volume. This phenomenon is especially 

 marked in jellies previously dried, which may absorb the first 

 portions of water with a force amounting to 1,000 atmospheres. 

 This force may be measured by the weight that the swelling 

 material is capable of lifting. Dry seeds are especially con- 

 venient for such experiments. In sweUing, they develop such 

 a pressure that they break glass containers. When placed in an 



