Control by Need for Minimum Heat 165 



Above the thermal threshold it is generally true that, the higher the 

 temperature, within limits, the shorter the period needed for the same 

 amount of development. This fact led to the belief that the heat re- 

 quirements for the growth of an organism from its youngest stage to 

 maturity, or for the completion of any other life process, might be cal- 

 culated quite mechanically as a "heat sum" (or "thermal constant") 

 expressed as "degree days" or "degree hours." The heat sum was 

 obtained by finding the threshold temperature for the process con- 

 cerned and summing the differences in degrees between this value 

 and the average temperature on each day until the completion of the 

 process. Thus the heat sum required for the flowering of corn plants 

 in Ohio was found to lie between 660 and 1050 degree days, basing 

 the calculation on the number of centigrade degrees above 6°C 

 recorded for the temperature each day. The lengths of the life his- 

 tories of many insects have been shown to depend upon a relatively 

 constant temperature sum over a considerable range of temperatures, 

 and hence of velocities of development (Allee et al., 1949, Ch. 6). 



Dala from Atlas of American Agriculture, U. S. Department of Agriculture, 1936 



Fig. 5.14. Average number of days between last killing frost in spring and first 



killing frost in fall. 



Although the results of temperature summation agree generally with 

 the fact that a given type of plant or animal requires longer to mature 

 in a cold than in a warm climate, closer scrutiny shows that, in many 

 species at least, temperature dependencies are far more involved than 



