PROTOPLASM 37 



nents; a fat phase, consisting of fats and fat-soluble components; and a 

 complex protein phase. Since the water phase is here the medium of 

 dispersion protoplasm is classed as a "hydrosol" in its ordinary state, 

 or as a "hydrogel" in the set condition. There are doubtless additional 

 minor phases present, protoplasm being in reality a "complex polyphase 

 colloidal system." 



The presence of water in protoplasm is a matter of fundamental im- 

 portance. As emphatically stated by Henderson (1913), "... the 

 physiologist has found that water is invariably the principal constitu- 

 ent of active living organisms. Water is ingested in greater amounts 

 than all other substances combined, and it is no less the chief excretion. 

 It is the vehicle of the principal foods and excretion products, for most 

 of these are dissolved as they enter or leave the body [across the wall of 

 the intestine and across the epithelia of kidneys, lungs, and sweat glands]. 

 Indeed, as clearer ideas of the physico-chemical organization of proto- 

 plasm have developed it has become evident that the organism itself is 

 essentially an aqueous solution in which are spread out colloidal sub- 

 stances of great complexity [Bechhold 1912]. As a result of these condi- 

 tions there is hardly a physiological process in which water is not of 

 fundamental importance" (pp. 75-77). 



The amount of water in protoplasm varies greatly under different 

 conditions, but normally it is present in large proportions. It makes up 

 85 to 95 per cent of the weight of actively streaming protoplasm such as 

 is seen in Elodea and Tradescantia, and in actively functioning cells it 

 rarely drops below 70 per cent. In dry spores, however, it may be 

 reduced to 10 or 15 per cent, in which case the protoplasm becomes very 

 viscous. The percentage differs constantly in different parts of the cell; 

 nucleus, cytoplasm, and plastids, though all are composed primarily of 

 protoplasm, contain very different amounts of water. Since active pro- 

 toplasm is a liquid, the phenomena of surface tension and other properties 

 of liquids must enter largely into explanations of its behavior. 



The colloidal nature of protoplasm is manifested in many of its prop- 

 erties. Its power of adsorption, which lies at the basis of many cell 

 reactions and certain staining processes, is similar to that of other 

 colloids. Protoplasm, like other colloids, is semi-permeable : a semi-per- 

 meable region is probably present wherever protoplasm comes in contact 

 with other substances, such as water; and the permeability of a vacuolate 

 cell is in general the resultant of the permeabilities of the ectoplast, 

 cytoplasm, and tonoplast. 



Protoplasm shows most strikingly its colloidal character in the changes 

 of physical state which it undergoes as the effects of variations in the 

 external conditions. The alterations due to changes in temperature will 

 serve for illustration. Above a certain temperature the colloid gelatin 

 exists in the sol state it is a hydrosol. If the temperature is sufficiently 



