1903.] Temperature and Carbon-Dioxide Assimilation, 355 



" On the Effect of Temperature on Carbon-Dioxide Assimilation." 

 By GABRIELLE L. C. MATTHAEI. Communicated by F. DARWIN, 

 F.RS. Keceived July 29, 1903. 



(Abstract.) 



1. The CCVassimilation of single cherry-laurel leaves has been 

 determined through a range of temperature from - 6 to 45 C. The 

 amount of C0 2 assimilated has been arrived at by the difference 

 between the C0 2 -content of a current of air before and after passing 

 through the illuminated glass chamber containing the leaf. 



At each temperature illumination of several different intensities has 

 been employed in order to make certain that the amount of assimilation 

 is not being limited by insufficient light. This has been neglected by 

 previous investigators, and quite erroneous results have been put 

 forward. 



Taking this precaution, there is obtained for each temperature a 

 maximal assimilation specific to tlwt temperature. The amount of light 

 required to produce the specific maximal assimilation varies directly 

 with the magnitude of the maximum. When this is once reached, 

 further increase in the illumination or in the amount of C0 2 supplied 

 produces no longer any augmentation of the assimilation. 



2. When a leaf is exposed to light of high intensity the absorbed 

 excess of radiation raises the temperature of the leaf above that 

 indicated by an adjacent thermometer. Under the conditions holding 

 in these experiments, this excess sometimes amounted to as much as 

 10 C. It became, therefore, absolutely essential to know the effective 

 internal temperature of the leaf. This has been determined by 

 inserting a very fine thermo-electric junction into the substance of the 

 leaf, and determining the temperature by means of a galvanometer at 

 intervals throughout the assimilation experiment. 



3. Care was taken that the leaves employed in these experiments 

 should be all initially in the same condition, since ancillary researches 

 had shown that differences in previous nutrition and temperature may 

 be disturbing factors in the results obtained. The leaves to be 

 employed were cut some time previously and kept, as a preliminary, 

 under similar conditions of illumination and temperature. 



In the course of the experiments it came out clearly that there are 

 marked seasonal variations in the absolute assimilatory power of 

 cherry -laurel leaves from the same individual plant. 



4. For determining the "real assimilation" under any conditions a 

 valuation of the concurrent respiratory C0 2 -production is essential. 

 At low temperatures the respiration is small in proportion to the 

 assimilation, and slight errors in valuation are not significant. The 



2 C 2 



