18 PLANT PHYSIOLOGY 



free unbound energy which could be used for doing work is always 

 getting less and less as it approaches a minimum, while the bound 

 energy which can no longer be used (or entropy) is progressing 

 towards a maximum. Surface tension is a form of energy and, in 

 accordance with this principle, anything which will lower the 

 surface tension of a solution will do so and will collect at the sur- 

 face. The large surfaces present on colloids give enormous room 

 for these changes in surface energy to occur, and it is probable 

 that the peculiarities of the plasma membrane and of the surface 

 layers of all the structures of the cell are due in part to the phe- 

 nomena which take place at surfaces. 



The Electrical Charge. — Colloids are generally charged pos- 

 itively or negatively, i. e., they carry either positive or negative 

 electrical charges. Most colloids such as colloidal gold, platinum, 

 caseinogen, charcoal, and clay are negatively charged when in 

 water, but others like the hydroxides of aluminum and iron are 

 positive. Particles of protoplasm seem to be negatively charged. 



The charge which a colloid carries is directly related to the 

 fact that it may be precipitated out of solution by salts or electro- 

 lytes. Solutions which contain charged ions will precipitate elec- 

 trolytes of an opposite charge. In a similar way a colloid may be 

 precipitated by a colloid of the opposite sign. If a solution con- 

 tains colloids of the same kind of charge, the repelling effects of 

 the similarly charged particles will help to keep the particles sus- 

 pended in the solution; but if colloids of an opposite sign are put 

 into the solution, the charges will be neutralized and neither one 

 will continue in suspension. Suspensoids are more sensitive to 

 electrolytes than emulsoids. 



Summary. — An elementary text is not the place for a complete 

 discussion of colloids, but enough has been said to show their 

 importance in any study of physiological phenomena. Proto- 

 plasm is a colloidal sol mostly of liquids in liquids, but there are 

 also fibrous and other solid particles present, giving it some of the 

 properties of a suspensoid; and it may at certain times even be a 

 gel. It certainly possesses some gel characteristics, among which 

 are its elasticity and rigidity. In all of the problems to be discussed 

 in the following pages, these properties must ever be borne in 

 mind; otherwise an intelligent study of the cell and its behavior 

 is impossible. 



