-58- 



It has been custortiery to discuss tempera tixre 

 coefficients as though they v/ere constant for each process, 

 and "van't Hoff's*^ principle in this connection has been 

 stated to the effect that chemical processes have a value 

 of C /ao\ rb out 2.0 or 2.5. As Pavfcett has emphasized, 

 however, the value of O /ld yvaries in magnitude, for any- 

 process, from infinity to zero. and the process is best 

 characterized (as toti.ts teraperat^^re relations) by showing 

 just how this variation occurs. For the growth data here 

 considered this is readily shovin by. a graph such as that 

 presented in figure 2, in v;hich the several temperature 

 ranges are presented as abscissas and the coefficient values 

 are shown by ordinates. Inspection of this graph shows that the 

 temperature coefficients for the shoot growth of these seedlings 

 follows the general law for such coefficients. For lov/ temper- 

 ature intervals the coefficient value is of cotirse infinite, 

 and for high intervals it is zero. The invervening values 

 /factually shown by fig. 2 ^^ vary from 4.5 (13 - 23 Jto 0.9 

 (25 -35°). AS to the Vant Eoff principle, like all other 



A 



processes (whether physical or chemical), this one of shoot 

 growth shov;s one interval for which the value of ^Aoyis about 



2.5; in this particular case this interval is about that from 



o o 

 15 to 25 . If attention were confined to the temperattire 



o o 



range from about 15 to about 27.5 the conclusion might be 



reached that the coefficient here considered has a value bf 



about 2 to 2.5. But the important feature to be considered is 



