

INFLUENCE OF TEMPERATURE 23 



limiting factor is operating, possibly connected with the 

 accumulation of the products of carbon assimilation (p. 35). 

 It was also found that immature and old leaves absorbed 

 carbon dioxide at a lesser rate than leaves of an intermediate 

 age. 



TEMPERATURE. 



The statement that chemical change is profoundly influ- 

 enced by temperature needs no elaboration : in the majority of 

 instances an increased temperature accelerates a reaction, 

 examples in which the contrary obtains are very few. Van't 

 Hoffs Law states that for every rise in temperature of 10 C. 

 the reaction is increased at a definite rate, in general terms 

 doubled or trebled, the precise value of which is specific to the 

 reaction.* The plant, however, is not a test-tube but a very 

 complex system of reacting substances, wherefore it is only in 

 experiments most carefully controlled and skilfully conducted 

 that approaches to the mathematical preciseness of well 

 ascertained physico-chemical laws will obtain. 



Long has it been known that an increased temperature 

 results in an increased carbon assimilation, but it was not 

 before F. F. Blackman's experimental researches on vegetable 

 assimilation and respiration that the subject was really criti- 

 cally examined. Matthaei f at the outset of her work on 

 the effect of temperature on carbon assimilation found that 

 in addition to the influential external conditions there is an 

 important internal condition of a plant or plant member, the 

 result of previous treatment such as excess of food, starvation, 

 and change in temperature. This has a most important 

 bearing on experimental results, a fact previously overlooked, 

 so that in order to have comparable results it is essential that 

 the previous history of all the material employed should be the 

 same. 



In the case of Prunus laurocerasus it was found that for 

 each temperature to which the leaves were subjected there is 

 a definite amount of carbon assimilation, the maximal assimil- 

 ation for that temperature, which cannot be exceeded and 



* This factor is termed the temperature coefficient and is represented by the 

 symbol K with a number attached indicating the number of degrees concerned, 

 e.g. K 10 . 



f Matthaei : " Phil. Trans. Roy. Soc.," Lond., B. 1904, 197, 47. 



