THE PHYSIOLOGICAL ZERO: AN EXPLANATION OF THE 

 DEPARTURE FROM THE LINEAR GRAPH OF 

 REACTION RATE VALUES AT THE 

 LOWER TEMPERATURES.* 



By JOSEPH KRAFKA, Jr. 



With the Collaboration of R. P. Stevens and David F. Barrow. 



(From the Zoological Laboratory of the University of Georgia, Athens.) 



(Received for publication, January 25, 1921.) 



Considerable controversy has arisen between investigators dealing 

 with the influence of temperature upon physiological reaction rates. 

 Some reactions are decidedly exponential, others clearly linear func- 

 tions of the temperature. To the first class belong the results of 

 Snyder (1913), on the rate of beat of the mammaHan heart and those 

 of Loeb and Northrop (1917) on the length of Hfe of Drosophila. 

 The work of Sanderson (1910), Peairs (1914) and others on insect 

 development and that of Krogh (1914) on general metabolism estab- 

 hsh the latter relationships. 



Marked deviations from the linear curves have been noted at the 

 upper and lower temperatures, making necessary the quahfying clause 

 that the linear relations only held between the normal limits of growth. 

 The departures at the upper temperatures have received an expla- 

 nation. It is generally conceded that the linear is a modified expo- 

 nential whose secondary factor increases more rapidly than the primary 

 one at the higher temperatures. 



The departures at the lower temperatures have only been noted 

 in connection with methods for the calculation of the physiological 

 zero. Thus if we were to apply Peairs' method for the determination 

 of the physiological zero to the development of Drosophila melano- 

 gaster it would come at about 8°C. in the work of Loeb and Northrop 

 (1917), and at 10°C. in my own experiments (Krafka (1919-20)). 



* Contribution from the Zoological Laboratory of the University of Georgia, 

 No. 1. 



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