SURFACE TENSION 165 



surface tension of at least 1,000 dynes per centimeter at the tip 

 of the pseudopods would be necessary if the pseudopods of an 

 amoeba are to have the same cutting capacity as the glass fiber. 

 The surface tension of protoplasm, according to estimations by 

 Czapek, is two-thirds that of water, or approximately 50 dynes 

 per centimeter. Surface tension is, therefore, according to Mast 

 and Root, probably at best an insignificant factor in the process 

 of feeding by Amoeba. This is probably quite true, yet surface- 

 tension changes may be involved in other ways than by merely 

 pinching in two. The cutting may be preceded 

 by digestion of the surface of the paramoecium. 



Far more complicated vital phenomena than the 

 movement and feeding of Amoeba have been attri- 

 buted to surface tension. But before considering 

 these, about which we must profess almost complete 

 ignorance, let us see if there really are one or two 

 processes associated with life which are indubitably 

 the direct expression of surface forces. The sim- Fig~^.— A 

 plest conceivable event which may be due to sur- budding yeast 

 face tension is the rounding up of a droplet of 

 protoplasm. The egg of a starfish or seaweed is round probably 

 because of surface forces. D'Arcy Thompson says that when a 

 "bud" appears on yeast (Fig. 98), it does so because at a certain 

 part of the cell surface the tension has diminished; the area at 

 that portion expands, and the bud will be rounded off into a more 

 or less spherical form. But he points out that the parent yeast 

 cell is not round and that it is only the incipient cell wall which 

 behaves like a fluid drop. The important point is not how an 

 asymmetrical shape is maintained but how it is acquired. An 

 ellipsoidal form, like that of an old yeast cell, is maintained 

 because the wall is a rigid gel. The lack of symmetry was 

 acquired while the wall was still fluid or plastic, when surface- 

 tension forces could therefore still operate. Structural features 

 in protoplasm, such as will account for elastic qualities (page 

 247), may be responsible for most protoplasmic phenomena which 

 have been attributed to surface tension. The laws of surface ten- 

 sion are based on the behavior of pure liquids and true solutions. 

 Protoplasm is certainly not the former, and there is little reason 

 to believe that it is the latter. When a droplet of water or oil 

 rounds up, surface tension is responsible, but a droplet of proto- 



