Ill 



THE EFFECT OF TEMPEKATURE 



109 



somewhere about 26° C. (or say 77° F.), or about the temperature 

 of a warm summer's day ; while it is found, very naturally, to be 

 considerably higher in the case of plants such as the melon or the 

 maize, which are at home in warmer regions that our own. 



In a large number of physical phenomena, and in a very marked 

 degree in all chemical reactions, it is found that rate of action is 

 affected, and for the most part accelerated, by rise of temperature ; 



14°16 18 20 22 24 26 28' 30 32 34 36 38 40° 



Temp. 



Fig. 25. Relation of rate of growth to temperature in certain plants. 

 (From Sachs's data.) 



and this effect of temperature tends to follow a definite "ex- 

 ponential" law, which holds good within a considerable range of 

 temperature, but is altered or departed from when we pass beyond 

 certain normal hmits. The law, as laid down by van't Hoff for 

 chemical reactions, is, that for an interval of n degrees the velocity 

 varies as x'^, x being called the "temperature coefficient"* for the 

 reaction in question. 



* For various instances of a "temperature coefficient" in physiological pro- 

 cesses, see Kanitz, Zeitschr. f. Elektrochemie, 1907, p. 707; Biol. Centralhl. xxvii, 

 p. 11, 1907; Hertzog, R. 0., Temperatureinfluss auf die Entwicklungsgesch- 

 windigkeit der Organismen, Zeitschr. f. Elektrochemie, xi, p 820, 1905; Krogh, 



