Presidential address. 



897 



To this final process of katabolism there could be no greater contrast than 

 the first step of anabolism, the assimilation of carbon by the protoplasm as a 

 result of photosynthesis. We must therefore next inquire what is the relation 

 of this process to temperature. 



This question is not so simple, as leaves cannot satisfactorily maintain the high 

 rate of assimilation that high temperatures allow. The facts of the case were 

 clearly worked out by Miss Matthsei,^ the rate of assimilation by cherry-laurel 

 leaves being measured from —6° C. to +42° C. Up to 37° C. the curve rose at 

 first gently and then more and more steeply, but on calculating out the values it 

 is found that the acceleration for successive rises of 10° C. becomes less and less. 

 Between 9° C. and 19° C. the increase is 2'1 times, the highest coefficient measured, 

 and exactly the same coefficient as for respiration in this plant, which in itself 

 is a striking point, seeing how different the processes are. (See fig. 5, Assim.) 



0' 10° ao' 30"' 40' SO'C^U. 

 Fig. 5. 



The decrease of the coefficient with successive rises is a state of things which is 

 quite general among non-vital reactions. A critical consideration of the matter 

 leads one to the conclusion, however, that this failure to keep up the temperature 

 acceleration is really due to secondary causes, as is also the appearance of an 

 optimum at about 38° C. Some of these causes have been discussed by me else- 

 where," and I hope to bring a new aspect of the matter before the Section in a 

 separate communication. The conclusion formerly come to was that probably 

 in its initial stages assimilation at these very high temperatures started at the 

 full value indicated by a theoretically constant coefficient, but that the proto- 

 plasm was unable to keep up the velocity, and the rate declined. It must be 

 borne in mind here that quite probably no chloroplast since the first appearance 

 of green cells upon the earth had ever been called upon for anything like such a 

 gastronomic effort as these cherry-laurel leaves in question. It is not to be 



' P?dl. Trans. Boy. loc, Ser. B, vol. cxcvii. 1904. 



2 • Optima and Limiting Factors,' Annals of Botanv, vol xix April 1905. 



1908, 3m 



