Growth and Form 101 



3. The growth of one part of an animal can be and usually is con- 

 trolled by the activities of another part. A prime example of this is the 

 dependence of many tissues upon secretions of the pituitary gland. 



The Limitations of Size and Form 



There are obviously great variations in the sizes of living things, 

 particularly between different species but often even within them. Many 

 factors govern size. Among them are the following: 



1. Genetic constitution. Mendel did some of his first experiments 

 with the dwarf and giant varieties of pea plants and showed that the two 

 differed by a single pair of genes. Such genetic determinants act by 

 affecting the rates of critical metabolic processes. 



2. Nutrient supply and toxicity. As shown in the previous section, 

 the growth of populations is limited by these factors. The same is true of 

 the growth of a single cell or a single multicellular organism. Any environ- 

 mental condition that affects either factor can also affect size and form. 



3. The ratio of surface to volume. Imagine an organism, perfectly 

 spherical in shape, which starts life with a radius of 1 centimeter and 

 grows to a radius of 10. Being spherical, its volume would be %Jir^ and 

 its surface 4jtr-. At the beginning, the volume would have totaled % X 

 3.14 X (1)^ = % X 3.14 X 1 = 4.2 cubic centimeters (cm^). After the 

 growth period, the volume would be % X 3.14 X (10)^ = ys X 3.14 X 

 1000 or 4200 cm^. Thus, the volume of the organism would have increased 

 by 4200/4.2 = 1000 times. The surface at the beginning would have been 

 4 X 3.14 X (1)- or 12.56 square centimeters (cm-) and at the end 4 X 

 3.14 X ( 10)2 = 4 X 3 14 y 100 or 1256 cm-. Thus, its surface would have 

 increased by 1256/12.56 or only 100 times. In other words, when an organ- 

 ism increases in size, the volume increases much faster than its available 

 surface area. The point of this is that if an organism experiences a thou- 

 sandfold increase in protoplasmic volume, it is going to require a thou- 

 sandfold more food to sustain itself and it is going to have to get rid of a 

 thousandfold more waste products. But food enters and wastes leave 

 through the surface of this organism which, as we have seen, increases 

 much less than its volume. Consequently, this limits the size that the or- 

 ganism can attain. Different species have overcome this limitation by: 



1. Changing their shape in order to decrease the disparity between vol- 

 ume and surface as the two increase during growth. 



2. Using their food supply more efficiently or by devising ways to make 

 its entrance more rapid. 



3. Devising biochemical tricks for cutting down on the production of toxic 

 wastes or by speeding the exit of wastes out of the system. 



