54 TEXTBOOK OF ZOOLOGY 



come sol. This transfer of water may be due to chemical changes in 

 the dispersed particles or in the dispersion medium of the colloid. 

 The ability of protoplasm, because of its colloidal nature, to change 

 from sol to gel state and back to sol repeatedly is the basis of many 

 of the vital activities, such as utilization of food, disposal of waste, 

 and movement. 



Fundamental Properties or Activities of Protoplasm 



In addition to the general characteristics, there may be mentioned 

 and described briefly a number of important activities common to 

 all protoplasm. These properties are: 



1. Irritability, which refers to the capacity present in all proto- 

 plasm for responding to changes in environmental conditions, or 

 external stimuli. 



2. Conductivity refers to the fact that the impulses produced by 

 stimuli or irritations at one point in protoplasm are conducted to 

 other parts of not only a single cell but also to adjoining cells. 



3. Contractility, which is the power of contraction and relaxation 

 that is common to the substance of every cell. 



4. Metaholism, the process of continual exchange of food and fuel 

 materials being built into the protoplasm, while, at the same time, 

 materials there are being oxidized to liberate kinetic energy, such as 

 heat and movement, and produce waste by-products. 



5. Growth is recognized as any increase in volume. When the rate 

 of the building side of metabolism exceeds the oxidation rate in the 

 protoplasm, there is storage of materials in the mass of the protoplasm 

 and hence growth. All protoplasm has this capacity. 



6. Reproduction is the capacity for producing new individuals of 

 the same kind. All living organisms are capable of this by some 

 means. Simple cell division is the most primitive process of repro- 

 duction among animals. 



Consciousness, which refers to the awareness of one's own exist- 

 ence, is frequently given as a property of protoplasm. It is certain 

 that some protoplasm possesses consciousness, but evidence of this 

 quality is rather intangible. Spontaneity is also considered a prop- 

 erty of protoplasm by some. To be certain that the activity and 

 source of all reaction comes from within is likewise rather difficult 

 of definite proof, so this is simply mentioned here as another prop- 

 erty which is often listed. 



