INFLUENCE OF TEMPERATURE 123 



curve for the range of temperature - 2 C. to 29 C. and 

 resembles those of Kuijper for respiration. Above 29 C. there 

 is so much fluctuation that relationship cannot be expressed 

 in a single curve, wherefore for each higher temperature a 

 different curve must be made to express the rate of growth 

 in successive periods of time. This is owing to the operation 

 of one or more of those imperfectly known factors termed by 

 F. F. Blackman the time factor. For instance, at 30 and 3 5 C. 

 the rate of growth in the first ten minutes is the highest at- 

 tained, in the first half hour there is a rapid fall followed by a 

 recovery marked by a rise to a second maximum, after which 

 there is a gradual fall. At 40 C., the decrease in growth 

 rate is uniform and rapid, there being no recovery.* As in 

 the cress, observed by Talma, so in the pea ; the coefficient 

 for a rise in the temperature of ten degrees shows a distinct 

 falling off as the temperature rises, and, according to the 

 observations of Leitch, it is only between 10 and 22 that 

 the coefficient value lies between 2 and 3, for which reason 

 the complete curve is not regarded as a van't Hoff curve. 

 The extremes of measurable growth was observed at - 2 

 and 44*5 C., the highest rate being at 30-3 C. With re- 

 ference to these observations, Leitch distinguishes four cardinal 

 points : the minimum temperature, the maximum temperature, 

 the optimum temperature and the maximum rate temperature. 

 The minimum temperature for any physiological process is 

 the lowest temperature at which the process 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 ; under such conditions some bio- 

 chemical reactions 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 tempera- 

 ture. Such a contingency may interfere with the supply of 



* See also Sierp : " Ber. deut. hot. Gesells.," 1918, 35, 3 ; *' Bot. Zentrbl.," 

 1920,40, 433- 



