ASSIMILATION OF CARBON 25 



filament, under the microscope, it is seen that the movement of the bacteria is 

 renewed in the neighborhood of both of the main chlorophyll absorption bands 

 (Fig. 14), being especially pronounced in the red and appreciably weaker in the 

 blue. It is only in the spectral regions thus indicated, therefore, that an evolu- 

 tion of oxygen occurs, to which the bacteria respond. 



The degree of difference between the efficiencies of the blue and red spectral 

 regions was established by Timiriazev. 1 For this purpose he divided the 

 spectrum into two equal parts by means of a cylindrical lens and a prism with 

 a very small angle of refraction. Flat-sided glass tubes containing pieces of 

 leaves of equal area were placed in the bright bands of blue and yellow light 

 thus obtained, and a gas analysis of the tube contents was made after three- 

 quarters of an hour or an hour. If the intensity of carbon dioxide decomposi- 

 tion in the less refrangible (red-yellow) light be taken as 100, then the corre- 

 sponding intensity in the more refrangible (blue) light is 54. Thus the light 

 absorbed by the leaves in the blue half of the spectrum is only about half as 

 effective as that absorbed in the other half. The absorption spectrum of the 

 leaves used in Timiriazev's experiment is presented in Fig. 15. It must be 

 noted, however, that the two absorption bands are not of equal width, the one 

 in the blue-violet region of the normal spectrum being more than three times 

 as wide as the band between B and C. If each of the ratios mentioned above is 

 divided by the breadth of the corresponding effective absorption band, there 

 is obtained for an average wave-length of the red region, 100, and for a similar 

 average in the blue-violet, 14, a relation which is graphically represented in 

 Fig. 15. Thus red light is relatively much more effective than blue-violet light. 

 How can this difference be explained? Obviously the explanation is to be found 

 in a consideration of the energy of the different wave-lengths expressed in 

 terms of their respective heat values, and (as will be seen from comparison of 

 the curve of decomposition of carbon dioxide with the Langley curve, AB, 

 representing the heating effect of the various parts of the solar spectrum) 

 both of these increase in the same direction. So the blue and violet rays have 

 only a comparatively slight effect in the decomposition of carbon dioxide, be- 

 cause, even though they are absorbed by chlorophyll, they represent only a very 

 small amount of energy." 



The dependence of the process of decomposition of carbon dioxide upon the 

 energy of the light rays was demonstrated in a still more detailed manner by 

 the experiments of Rikhter. 2 Only light that is absorbed can decompose 

 carbon dioxide, and those wave-lengths of the absorbed light are most effective 



1 Timiriazev, C, Photochemische Wirkung der am Rande des sichtbaren Spektrums liegenden Strahlen. 

 1893. (Russian.)* 



2 Richter, Andre, Etude sur la photosynthese, et sur l'absorption par la feuille verte des rayons de 

 differentes longueurs d'onde. Rev. g6n. bot. 14: 151-169. 211-218. 1902. Kohl, 1897- [See p. 5, 

 note 1.] 



These statements apply to leaves and should not be interpreted as necessarily applying 

 to chlorophyll, for leaves contain carotin, etc., which surely affect their power to absorb 

 radiation. Some references on sunlight have been given in note 1, p. 22. See also: 

 Iwanowski, D., Ein Beitrag zur physiologischen Theorie des Chlorophylls. Ber. Deutsch. 

 Bot. Ges. 32 : 433-447. 1914.— Ed. 



