170 Regulation of Size in Unicellular Organisms 



though not spherical their proportions are not main- 

 tained constant as the individual enlarges. 



Electrical charge. Another force of little impor- 

 tance in limiting the form and size of large organisms, 

 but of great importance to micro-organisms, is electri- 

 cal potential. Whereas to a frog differences of electri- 

 cal potential may have to do with the equilibria of sub- 

 stances between body and medium, in a bacterium they 

 have to do not only with these local processes but also 

 with bodily individuality and with resistance to many 

 sorts of death-producers, which depend upon stability 

 in suspension. In other words, a potential may com- 

 pensate for a high force of cohesion or a small surface 

 tension. A potential is one more means by which the 

 balance of surface and internal forces may be adjusted. 



It is known that the preservation of bodily integrity 

 by unicellular organisms does not depend so much 

 upon chemical differences between body and medium 

 as upon physical boundaries. One of the ways in 

 which lethal substances produce their effects is by 

 modifying the electrical properties of the surface. This 

 was shown extensively by the observations of Hardy 

 and Harvey ('11), who measured the potential by ca- 

 taphoresis of the organisms which had been treated in 

 various ways. It is hardly enough to realize that elec- 

 trical properties are significant in organisms of small 

 size; modifications of size must be sought in correla- 

 tion with changes of potential. 



