156 PROTOPLASM 



There occur in nature a number of examples of concentric 

 bands which look suspiciously like Liesegang precipitates. This 

 is true of the annual rings of trees; the rings formed by fungi, 

 especially when grown in culture {F, Fig. 92); and the extra- 

 ordinary banding caused by fungi on the trunks (bark) of trees. 

 These processes are not to be confused with the Liesegang 

 phenomenon; they are quite distinct from it, except that both 

 are examples of the widespread tendency of natural processes to 

 function rhythmically. 



The Gel Qualities of Protoplasm. — Many of the properties 

 and processes of protoplasm are typical of gels. Cells take up 

 water in part by imbibition. Wood swells by imbibition. 

 These are properties of gels. The control of the entrance of salts 

 into the cell is determined, in part, by the outer layer of proto- 

 plasm which is a jelly. The elastic properties of protoplasm, its 

 coagulation, and its adhesive qualities are properties of gels. 

 (W. H. Lewis says that if it were not for the adhesive quality of 

 cell surfaces we should all fall to pieces!) Adsorption, which 

 plays so important a part in life, often acts in the minute spaces 

 of a gel. Changes in the consistency of protoplasm, whether 

 slow ones in viscosity or sudden ones in thixotropy, take place in 

 going to and from the gel state. 



That structure of protoplasm upon which its organization 

 as a living system primarily depends is essentially the structure 

 of a jelly. Protoplasm possesses some of the properties of liquids 

 — of a sol — thus, it flows and rounds up under the influence of 

 surface tension, but its gel qualities are more marked. Though 

 often fluid, and though superficially an emulsion, protoplasm is 

 primarily and fundamentally a lyophilic colloidal system, that is 

 to say, a jelly. 



