ASSIMILATION OF CAEBON 25 



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 J- 

 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 stiU more detailed manner by 

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

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

 which furnish the greatest amount of heat energy. Rikhter used solutions of 

 potassium dichromate, ammoniacal copper oxide and potassium permanganate 

 as light-filters. The plant received the following relative amounts of light when 

 placed behind the various filters: 



Potassium Dicero- Ammoniacal Copper Potassium Perman- 

 ^Vater J' 



mate Solution oxide Solution ganate Solution 



1000 491 177 233.0 



100 36 47. S 



The corresponding relative rates of carbon dioxide decomposition behind the 

 same light screens proved to be, on the average, as follows: 



Water 



The numbers in the two series agree so closely as to suggest that the amount 

 of photosynthetic work accomplished by a ray of light is proportional to the 

 amount of energy absorbed by the leaf, and is independent of the wave length 

 of the ray and of its position in the spectruni.^ 



Carbon dioxide is thus seen to be decomposed most rapidly in green plants by 



' Ricliter, Andrb, fitude sur la photosynth^se, et sur I'absorption par la feuille verte des rayons de 

 diff^rentes longueurs d'onde. Rev. gto. bot. 14: 151-169, 211-218. 1902. Zohl, 1897. [See p. i, 

 note 5.] 



2 See also: Eniep, H., and Minder, F., Ueber den Einfluss versohiedenfarbigen Lichtes auf die Kohlen- 

 saureassimilation. Zeitsch. Bot. 1 : 619-650. 1909. [Purlewitscli, K., Untersuchungen uber Photosyn- 

 these. Jahrb. wiss. Bot. S.I : 210-254- I9i3-] 



° 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 i, p. 22. See also: 

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

 Bot. Ges. 32: 433-447. 1914. — Ed. 



