Maximum Temperatures 143 



above 20°C (Went, 1950). Many Arctic fish do not venture south 

 into waters as warm as 10°C, but this same temperature would be far 

 too cold for most tropical fish. 



If the tissues do not freeze, why are these animals and plants killed 

 by moderately low temperatures? Most chemical reactions are slowed 

 down by lowering temperature and eventually stop. The cessation 

 of any one of the vital processes will cause the death of the organism. 

 However, the various processes going on in the body come to a stop 

 at various points on the temperature scale. No one biological zero 

 exists for all reactions even within the same individual. Under some 

 circumstances the organism may die when all the vital processes are 

 still going. The explanation may be that the different reactions are 

 slowed down by different amounts with the result that mutually de- 

 pendent processes get out of adjustment and cause the death of the 

 organism. 



The minimum temperature for the organism as a whole is thus de- 

 termined by the most susceptible of the vital processes. The lowest 

 temperature at which the organism can live indefinitely in an active 

 state is termed the minimum elective temperature. After a further 

 reduction of temperature the organism goes into chill coma. If too 

 long a period does not elapse before the organism is again warmed, it 

 will become active once more. The lowest temperature at which 

 survival is possible is called the minimum survival temperature. The 

 actual value of this temperature depends upon the period of exposure. 

 For example, the eggs and larvae of the fruit fly, Ceratitis capitata, 

 were killed at 7°C only after an exposure of 7 weeks, but death was 

 caused after 3 weeks at 4°C or after 2 weeks at 1°C. Accordingly, a 

 statement of the minimum survival temperature for an animal or a 

 plant has no meaning unless the period of exposure is also stated. 



Maximum Temperatures. At the other end of the temperature 

 scale the same sort of situation exists. The maximum elective tem- 

 perature is the greatest intensity of heat at which the species can live 

 indefinitely in the active state. The elective temperature range within 

 which the organism can carry on its active life and beyond which death 

 eventually results extends between the maximum and minimum effec- 

 tive temperatures. At higher temperatures the organism goes into 

 heat coma but will recover if restored before too long to cooler con- 

 ditions. For the maximum survival temperature the period of the ex- 

 posure of the organism to the specific heat conditions must be given. 

 All these thermal relations are illustrated for the house fly in Table 8. 



