96 INFLUENCE OF TEMPERATURE 



It is, therefore, in unicellular organisms, and in the earlier stages 

 of development of multicellular organisms, that the variation in 

 activity with alteration in temperature is most clearly seen. Here 

 it is found that at a certain minimal temperature the activity 

 just begins to be perceptible, as the temperature rises the activity 

 increases with it until an optimum point is reached, lying usually, 

 as in the case of the enzyme, a few degrees above the usual 

 temperature of the action of the organism, and beyond the 

 optimum temperature the life of the organism becomes again 

 more sluggish, its activities lessened, and if the high temperature 

 is maintained it dies. The optimum point of activity for the 

 living cell probably arises in the same manner as in the case of 

 the enzyme, by the simultaneous action of two opposing causes ; 

 viz. (1) the hastening of all chemical reactions by rise in the 

 temperature, (2) a similar hastening from the same cause of the 

 bye-reactions which lead to a using up and diminishing of the 

 cell substances which act as catalysts to these reactions. As 

 a result of this it follows that in the earlier stages of the range 

 of temperature, as the temperature rises, the chemical reactions 

 in the cell will increase in velocity, while as yet there will be no 

 appreciable destruction of the more stable cell substance, or of 

 the catalysts. But in the later stages, at the optimum point 

 and beyond it, destruction of catalysts, cellular enzymes, and cell 

 substance will also proceed at an ever-increasing rate, and although 

 the velocity of reaction of the catalysed reactions (under No. 1) 

 is still increasing, the diminution in catalyst more than out- 

 balances this, and the cell activities are lessened. 



The same differences are to be noted at the two extreme ends 

 of the range in the case of enzymes and living cells ; as the 

 temperature falls, the enzyme and living cell merely become dormant 

 and temporarily pass out of activity, but neither is killed unless 

 the fall in temperature is enormous compared to the rise in tempe- 

 rature which would cause total permanent loss of activity or death 

 upon the other side of the active range. Nor is the reason far to 

 seek ; the lower limit is reached by gradual fall in activity until 

 the zero point is reached, while the upper limit is reached by 

 gradual increase in activity, accompanied by gradual destruction 

 finally surpassing increase in activity, until the cell stops from 

 destruction in hyper-activity. 



The level at which living cells are rapidly destroyed by increased 



