PROTOPLASM 21 



5. Protoplasm is also exceedingly variable. There is a difference 

 chemically between the protoplasm of one species of animal or plant and 

 that of another. Moreover, there is also a difference between the forms 

 of protoplasm contained in the various structures found in the body of 

 each animal and plant. With all the individuality that exists in living 

 things it is conceivable that no two bits of living matter are ever precisely 

 alike. This characteristic is back of all the adjustments of living matter 

 to its environment. 



6. Protoplasm also undergoes an orderly sequence of chemical reac- 

 tions which we call metabolism, and as long as life is being manifested the 

 cycle of such reactions is being repeated over and over. 



28. Physical Characteristics of Protoplasm. — Protoplasm has the 

 following physical characteristics : 



1. It is viscid and gelatinous in consistency, differing in viscosity in 

 various forms of life, in the various structures of the body, and under 

 various conditions. 



2. Its texture, generally speaking, is more or less granular. 



3. It is colorless in pure form. All colors which it seems to possess are 

 due to the presence of colored bodies within the living substance. 



4. It is more or less translucent, being never perfectly transparent, 

 and this translucency gives to it in mass a grayish appearance. 



5. It is of the nature of an emulsoid, or colloidal emulsion, the various 

 substances of which it is composed being distributed through the disper- 

 sion medium and in the droplets of the disperse phase. Being colloidal in 

 character and being reversible, it is possible for water and substances in 

 solution to enter protoplasm from without, causing it to become more 

 fluid, or to pass out from it, resulting in its becoming firmer in consistency. 

 To the same fact and to the fact that the internal surface films in such an 

 emulsion may act like semipermeable membranes is due the possibility of 

 water and substances in solution passing in both directions through 

 the walls of the droplets, causing them to swell or to shrink. As they 

 swell and crowd together the whole tends to become a gel, and as they 

 shrink and move with greater ease in the more fluid dispersion medium 

 it tends to become a sol. This transfer of water may be the consequence 

 of chemical changes taking place in either the substance of one phase or 

 that of the other. 



This ability of protoplasm to change from sol to gel and back to sol 

 over and over again is behind many vital activities, including all move- 

 ment. The entrance of water and substances in solution into the mass at 

 certain times and the giving up of water and other substances in solution 

 at other times make possible the taking in of food and the giving out of 

 waste. The passage of materials through the internal films, which are 

 the walls of the droplets of the disperse phase, possibly plays a part in 



