INFLUENCE OF TEMPERATURE 191 



cardinal points : the minimum temperature, the maximum 

 temperature, the optimum temperature and the maximum rate 

 temperature. The minimum temperature for any physio- 

 logical process is the lowest temperature at which the pro- 

 cess takes place ; the maximum temperature is the highest 

 temperature at which the process takes place ; the optimum 

 temperature is the highest temperature at which there is no 

 time factor operating ; and the maximum rate temperature 

 is that temperature at which the process attains its highest 

 intensity. 



The fluctuations observed at higher temperatures may be due 

 to several factors ; in such conditions some biochemical re- 

 actions may be intensified but not necessarily at the same rate ; 

 and some may be inhibited in varying degrees. The operations 

 of enzymes, for example, are not affected in precisely the same 

 way by the same high degree of temperature. Such a contin- 

 gency may interfere with the supply of soluble food, and, con- 

 currently, the rapid accumulation of the products of respiratory 

 processes, which also are accelerated by an increased tempera- 

 ture, may have a toxic action. In a few words, up to a certain 

 degree a rise in temperature accelerates physiological actions 

 and here a van't Hoff curve may be expected : beyond 

 this degree, co-ordination becomes less and less, owing to 

 metabolic derangement. Priestley and Pearsall,* in their 

 examination of Leitch's results, point out that the growth 

 rate of radicles is dependent on the chemical reactions involved 

 in the merismatic activity of the growing point and on the 

 hydrolysis of the reserve food in the seed and the translo- 

 cation of the products to the growing parts. The situation 

 therefore is this : the increase in temperature accelerates 

 both growth and hydrolysis, but the merismatic tissue quickly 

 uses up the food immediately available, wherefore a decrease 

 in the rate of growth must ensue for that period of time taken 

 by the food materials in their translocation from the seed to 

 the apical regions of the root. The arrival of this food ac- 

 counts for the second maximum in Leitch's curves. Finally, 



* Priestley and Pearsall : " Ann. Bot.," 1922, 36, 239. 



