16 Regulation of Size in Unicellular Organisms 



The limit of vision through a microscope, due to the 

 length of light waves, is about 0.20 micron. It seems 

 as though the varieties of ultra-visible organisms could 

 not be very numerous. Some of the smallest organ- 

 isms which have been seen by direct vision are com- 

 pared in table 2. It is not wholly certain that there 

 exist organisms which cannot be seen. 



Table 2. 



Some Smallest Visible and Some Largest Known Species of Unicellular 

 Organisms. Volumes are Calculated as lXbXtXir/6. 



Relative sizes. In table 2 are given, for comparison 

 with the two species of minute bacteria, the smallest 

 known species of protozoan (Pseudomonas) and three 

 of the largest species of unicellular organisms, which 

 however are not the very largest. The body of one 

 Spirostomum then contains 4 x 10 10 times as much sub- 

 stance as the body of a Pseudomonas. Even in length 

 it is 9 x 10 4 times. The hugeness of the diversity in 

 volumes may be made clear by stating that an average 

 man has only 5 x 10 6 times the volume of a Spirosto- 

 mum. On the whole, there is as much diversity in size 

 among unicellular organisms, as there is between uni- 

 cellular organisms and any other group of organisms. 

 Their unicellular structure is clearly no explanation of 

 their size. 



