50 PROTOPLASM OF PROTOZOA 



ently differentiated; the contractile vacuole and other structures are 

 evanescent, at least in part. We have already mentioned the more ob- 

 vious physical characteristics of Amoeba protoplasm, which consists, as 

 does all protoplasm, of a complex heterogenous colloidal system, made 

 up of a suspension (granular, fibrillar, and alveolar) of many different 

 materials (fats, carbohydrates, and proteins) dispersed in a supporting 

 continuous liquid part. The greatest volume ingredient in protoplasm 

 is water; in solution in the water are various salts. Protoplasm may be 

 deprived of many of its visible inclusions without killing it, leaving 

 often a clear, colorless, optically homogenous hyaloplasm. Thus the 

 microscopic structure of protoplasm gives no direct evidence of the 

 finer submicroscopic structure. However, it is generally agreed that the 

 finer structure of protoplasm is dependent upon its colloidal nature, 

 which, because of the relatively large size of the particles present, results 

 in enormous intracellular surfaces. 



Then too, the taking up of water by colloids is influenced both by the 

 solutes present and by the previous history of the colloid itself (hys- 

 teresis). Colloids often have the property of changing reversibly from 

 a relatively liquid (sol) to a relatively solid condition (gel). A gel 

 has many of the properties of a solid, among them elasticity, apparently 

 due to its structure; but differs from a solid in that diffusion in gels of 

 low concentration is often the same as in a simple solution, and in that 

 chemical reactions often can occur at velocities unaffected by the gel 

 condition. Every organism is dependent upon the temporal and spatial 

 coordination of its chemical reactions, and this depends largely on the 

 degree of dispersion and kinetic activity, because these regulate re- 

 action velocities. Thus the organization of chemical events is due in 

 some way to the nature and architecture of the colloidal system in which 

 they occur. Therefore, biologists attempt to explain the physiological 

 action of various factors as influences on the colloids of protoplasm. 



Consistency 



It is a phenomenon of general observation that protoplasm flows, 

 but that it resists pressure. The former is a property commonly attributed 

 to liquids, the latter to solids. In liquids there is great internal mobility 

 of the molecules; this is essential to many physical activities, such as 



