117 



and an accumulation of carbon takes place in the plant, but 

 the colouring matter takes no direct and immediate part in 

 the process. 



c. How does this Theory of the Function of Chlorophyll 

 affect the Result of Researches on Assimilation ? — As the 

 oxidation and reduction-processes in green tissues do not rise 

 and fall similarly in changing light-intensities and colours, it 

 is necessary in all questions concerning assimilation to take 

 into consideration the amount of respiration and the extent 

 to which it is influenced by light. And 



1. As regards an optimum intensity of light for the decom- 

 position of carbonic acid. — This cannot be determined 

 simply by the amount of oxygen given off in light, for this 

 in all intensities is only the excess of oxygen exhaled over 

 what is inhaled. Both processes, that of exhalation and that 

 of inhalation of oxygen, are differently affected by light- 

 intensity and colour, and if they are not distinguished from 

 one another in a research the determination of the amount 

 of oxygen exhaled fixes only approximately the relation 

 between respiration and assimilation. It is possible in this 

 way to determine only the intensities in which one or other 

 process predominates, but not the amount of increase or 

 decrease of decomposition of carbonic acid. The amount of 

 oxygen exhaled, as has already been pointed out, is not an 

 exact measure of the decomposition of carbonic acid, for an 

 increase in brightness of illumination may bring an increase 

 in decomposition of carbonic acid with an apparent decrease 

 in the amount of oxygen given off (fig. 29, shows the ap- 

 proximation of the two curves in the highest intensities of 

 daylight) ; and in this lies the explanation of the smaller 

 amount of gas-evolution in direct sunlight than in bright 

 diffuse daylight. 



If now the exact carbon gain be sought instead of the 

 amount of decomposition of carbonic acid in light, the oxygen 

 exhalation again only gives an approximate result so long as 

 the quantitative relation of the carbonic acid exspired to the 

 oxygen inhaled in varying intensities is unknown. The 

 question is still further complicated by the unequal posses- 

 sion by the plants of chlorophyll, which must exercise an 

 equally varying influence upon respiration and assimilation. 



2. The relative energy of the different rays of the spectrum 

 in assimilation. — This cannot be determined by the amount 

 of gas given off in the different colours, because the absorp- 

 tion in the chlorophyll colouring matter, according to the 

 screen-theory here set forth, must modify the result. All 



