136 PHOTOSYNTHESIS 



observed will depend uixjii the time that has elapsed between the beginning 

 of the experiment and the determination of the rate. Ihis effect is known 

 as the "time factor" and is discussed in more detail in a later section of 

 this book. 



In Figure 13 is shown schematically the manner in which the time 

 factor afifects the rate of photosynthesis. At lower temperatures the rate 

 of photosynthesis is fairly constant for successive periods of time. Above 

 about 24° there is a constant falling off of the rate with time. The 

 exact temperature at which this begins very probably varies with differ- 

 ent leaves. In Figure 13 the broken lines 2, 3, 4, indicate rate of photo- 

 synthesis after the second, third and fourth hours. The result thus is a 

 shifting of the maximal rate to lower temperatures. 



As a result of theoretical considerations developed by lilackman ^^'^ 

 and others the influence of time on the optimal rate of photosynthesis has 

 been interpreted on the basis that there are two op|X)sed reactions in- 

 volved. This principle of superposition of two curves has been em- 

 ployed for a variety of reactions. ^^^ From Miss Matthaei's results it can 

 be determined that the temperature coefficient, Qio, for the rate of photo- 

 synthesis of leaves of the cherry laurel is 2.1 for temperatures between 

 5° and 25°. That is, between these temperatures the rate of photosynthe- 

 sis follows closely what is frequently called the van't Hoff rule. Above 

 and below these temperatures it does not follow this rule and, as has 

 been observed in other reactions of living organisms, when the limits of 

 temperatures are approached there is a great variation in the rates from 

 that expected from the van't Hoff rule. 



The type of curves obtained from a study of the effect of tempera- 

 ture on photosynthetic rate bears a strong similarity to the curves of the 

 effect of temperature on the rate of catalysis of a number of different 

 chemical reactions. The interpretation of these latter reactions have 

 been used to explain the results obtained with photosynthesis. More 

 particularly have the results of Tammann '^^ and of Duclaux ^^^ on the 

 effect of temperature on fermentation been taken as types of reactions 

 which find application to the photosynthesis problem. These workers have 

 shown that the relation of temperature to the rate of a fermentation re- 

 action depends upon two different factors, the temperature and the con- 

 centration of the ferment. The latter are thermolabile substances, i.e., 

 they are destroyed or inactivated at ordinary temperature and the rate 

 of this destruction increases with temperature. As the temperature of 

 the fermenting mixture is increased, the rate of the fermentation reaction 

 is accelerated at the same time; however, the ferment is also being de- 

 stroyed, as this cannot endure high temperatures for any length of time. 

 The latter reaction reduces the concentration of the ferment which re- 



"* Blackmail, Ann. of Bot., 19, 282 (1905). Kanitz, "Temperatur und Leben- 

 svorgange," Berlin, 1915, p. 16. Jost, Biol. Zcntralb., 26, 225 (1906). 

 "'Bredig, G., Ergeh. Physiol., 1, I, 198 (1902). 

 "'Tammann, Zeits. phvsiol. chem., 16, 317 (1892). 

 "* Duclaux, "Traite de microbiologic," 2, 193, Pans, 1899. 



