The Temperature Relation 



405 



environment is constant, there is a theoretical thermal con- 

 stant, and it is of sufficient importance to receive some 

 practical consideration. 1 



243. Heat units and germination. If the number of 

 heat units required in order to bring a plant to maturity 

 were at all constant, then the number requisite for any 

 phase of growth should likewise be more or less constant. 

 Some interesting data are available respecting germination, 

 and in the following table the time intervals are given for 

 germination at the temperatures indicated, and the heat 

 units may be readily computed : 



1 There are several methods of computing heat units. In each case it 

 is necessary to know the period of' growth in days and the daily mean 

 temperature during the growing period. With this data we may then 

 obtain the total heat units by multiplying the growth period by the daily 

 mean temperature. This method makes C. or 32 F. the basis. In the 



