PROTOPLASM 21 



in solution may not be ionised, e.g. glucose, or they may be 

 ionised into an-ions and cat-ions. Among the more 

 important of tlie cat-ions are potassium, calcium, and 

 sodium. The presence of such crystalloids free in true 

 solution chiefly determines the osmotic pressure of the mass 

 of protoplasm, and hence this may vary from time to time 

 according to whether these substances are united to the 

 proteins or are free. 



The osmotic pressure in the protoplasm of a cell may be 

 ascertained by subjecting it to fluids of different osmotic 

 equivalents and determining whether it swells by the passage 

 of fluid inwards or shrinks by the passage of water outwards, 

 thus ascertaining the molecular concentration of the 

 surrounding fluid and so of the cell itself. This has been 

 called the method of Plastnolysis. The red cells of the 

 blood have a very definite osmotic pressure, and when 

 subjected to a fluid of lower osmotic pressure, they swell, 

 while in a fluid of higher osmotic pressure, they shrink. 

 This is called the metliod of Hcemolysis. 



IV. Protoplasmic Activity. 



This complex of substances called protoplasm is, during 

 life, in a constant state of active chemical change. All 

 its conditions make for great instability : its colloidal 

 nature, its demarcation from its surroundings as the result 

 of surface tension with adsorption, its frequent division into 

 innumerable vesicles separated from one another by very 

 unsubstantial and temporary septa ever changing as the result 

 of internal chemical changes, all of these combine to produce 

 a very labile condition. Thus oxidation may be going on in 

 one part of the mass, drawing oxygen from another, and thus 

 leading to a simultaneous process of reduction. Such a 

 mechanism is pregnant with possibilities as a transformer 

 of energy and as a producer of movement. 



