190 



CIVIC BIOLOGY 



the cell membranes is slow. The amount absorbed is pro- 

 portional to the absorbing surface exposed to the solution. 

 With these points in mind we may understand why the ac- 

 tive mechanisms in living things are so minute, for only in 

 this way are they able to present the largest possible sur- 

 face for both the escape ' of waste matters and the absorption 



of food. The diagram 

 on this page presents 

 these relations in sim- 

 ple form. A one-inch 

 cube is seen to have 

 six square inches of ab- 

 sorbing surface, while 

 in a ten-inch cube each 

 cubic inch has only six 

 tenths of one square 

 inch of surface. The 

 rate of absorption be- 

 ing the same, the 

 smaller cube could 

 absorb ten times as 

 fast as a similar bulk 

 of the larger cube. So 

 we see why the small- 

 est organisms may be 

 the most efficient in ab 

 sorbing food and have 

 the greatest power of 

 growth. It is estimated that a bacterium J^QQ- ^ a milli- 

 meter in diameter, which can double in size in twenty min- 

 utes, given food and suitable conditions, might grow to a 

 mass the size of the earth in about five days. A yeast plant, 

 which is much larger but which can double in thirty minutes, 

 might grow to a similar mass in about two weeks. How 



FIG. 91. Diagram to show relation of surface 

 to bulk in large and small organisms 



The law is: Bulk increases as the cube, while 

 surface for absorption increases only as the 

 square. Since bulk so rapidly outstrips surface, 

 this relation tends to limit the size of organisms, 

 and suggests one of the fundamental reasons 

 why minute organisms possess such phenom- 

 enal powers of growth and reproduction 



