Temperature. 69 



B. Temperature. 



From the researches of van't HofF it is well known that the 

 relation between temperature and the reaction velocity of a good 

 many chemical reactions can be expressed in a simple way, revealing 

 the fact that in many cases the reaction rate at moderate temper- 

 atures is increased 2 or 3 times for a rise of 10°C. On the other 

 hand, animal physiologists have shown that although a smooth 

 temperature-metabolism curve can be constructed which gives the 

 relation between temperature and respiration, yet this curve does 

 not obey the van't HofI rule. Similar curves have been obtained 

 for plant respiration by Kuijper (1910) and for one aspect of plant 

 growth (Leitch, 1916). Such a curve is shown in Fig. 7 and its 

 relation to true van't Hoff curves with different coefficients (Qm) 

 exhibited. Some consider such a curve as made up of portions of 

 several van't Hofif curves having different constants (Putter, 1914). 

 Krogh (1916) points out that it is not very probable from a priori 

 considerations that the van't Hoff rule should be followed, as we 

 have to do, not with a simple chemical reaction, but with a complex 

 series of reactions possibly taking place in a heterogeneous system. 

 And even if the difference between the heterogeneous system and 

 a system in solution could be neglected, yet the shape of the 

 curve would still be affected if a limiting factor were operative. 

 Thus oxygen pressure in the tissues might be a limiting factor. 



Owing to Blackman's recognition throughout his work of the 

 effect of limiting factors, our knowledge of the relation between 

 temperature and carbon assimilation is much clearer. It is recognised 

 by Blackman that in investigating the influence of temperature on 

 carbon assimilation, no other factor must be limiting the rate of the 

 process, as in such a case, the amount of the carbon assimilation is 

 simply dependent upon the value of the limiting factor and is not 

 related to the temperature. 



The influence of temperature on carbon assimilation is described 

 in two papers, one by Miss Matthaei (1904) and a second by 

 Blackman and Matthaei (1905). In these papers will be found a 

 general account of the apparatus and method used. Isolated leaves 

 of Cherry Laurel {Primus laurocerasns var. rotundifolia) were used 

 in most experiments, while for some, leaves of Helianthus tuherosus 

 were employed. The leaves were carefully selected and after 



