12 Regulation of Size in Unicellular Organisms 



stances, in turn, has to do with the amount of each 

 present at any moment. And the amount present is 

 a component of the total body substance present; 

 namely, the body size. The problem of size regula- 

 tion is obviously in an initial stage of study; body sizes 

 may be compared, though in complete ignorance of the 

 roll of the components, with the exception of slight 

 information concerning the one component, water. 



3. Some Theoretical Points 

 At the present time the consideration of various sim- 

 ple physical and geometrical factors is found to be sin- 

 gularly futile in solving the problem of the regulation 

 of size, for in no case has the role of any one fundamen- 

 tal factor been evaluated. It is worth while, however, 

 to mention a few of the likely factors in the hope that 

 their application may become, in course, justified. 



Similitude. The principle of similitude states that 

 similarly shaped and proportioned objects which differ 

 in size have significantly different properties. This 

 principle, and its application to biological matters, was 

 probably first fully recognized by Galileo (1638). He 

 stated, "In a diminishing solid the weight grows less 

 in proportion to the volume, and since the volume al- 

 ways diminishes more rapidly than the surface, when 

 the same shape is maintained, the weight must dimin- 

 ish more rapidly than the surface". The significance 

 of this relation depends upon what processes are limi- 

 ted by the surface and what properties depend on the 

 volume. Spencer ('66) believed that inertia and grav- 

 ity are the essential correlatives of volume, while cohe- 

 sion and absorption are limited by surface. Expendi- 

 ture in work increases as the cube of the linear dimen- 

 sion; supply or absorbing surface as the square. 



The sizes of micro-organisms must be chiefly depen- 

 dent upon a very different set of factors from the sizes 

 of large organisms. Intermolecular and surface forces 



