26 PHYSIOLOGY OF NUTRITION 



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 Dichro- Ammoniacal Copper Potassium Perman- 



Water mate Solution Oxide Solution ganate Solution 



iooo 491 177 2330 



100 36 47.5 



The corresponding relative rates of carbon dioxide decomposition behind the 

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



Potassium Dichro- Ammoniacal Copper Potassium Perman- 



Water mate Solution Oxide Solution ganate Solution 



iooo 494 168.0 249 



100 34.4 48 



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 spectrum. 1 



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

 the light rays between lines B and C. But when other pigments besides chloro- 

 phyll are present, the maximum of this decomposition may fall in another part 

 of the spectrum. 2 In the Cyanophyceae the maximum occurs at D; the brown 

 algae show a maximum between D and E, although the decomposition between 

 B and C is here almost as great; finally, the red algae have a maximum between 

 D and E also, but the decomposition between B and C is here very weak. 

 These facts are in agreement with the distribution of the various algae, accord- 

 ing to depth, in the ocean; while the surface layer of water is mainly inhabited 

 by green algae, the red forms are found at very great depths. Spectroscopic 

 investigations have shown that red light, which is essential to green algae, is 

 quickly absorbed by water and that this light is entirely absent at no great 

 distance below the surface. On the other hand, the green and blue rays, which 

 are absorbed by the red algae, attain great depths. 



According to Engelmann, 3 plants that contain no chlorophyll may also 

 decompose carbon dioxide, provided they contain another pigment; as, for in- 

 stance, the purple bacteria. p 



Engelmann's theory of complementary pigments found confirmation in the 

 interesting researches of Gaidukov 4 upon the influence of colored light upon 



1 See also: Kniep, H., and Minder, F., Ueber den Einfluss verschiedenfarbigen Lichtes auf die Kohlen- 

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

 these. Jahrb. wiss. Bot. 53 : 210-254. 1013.] 



2 Engelmann, Th. W., Farbe und Assimilation. Bot. Zeitg. 41 : 1-13, 17-29. 1883. 



3 Engelmann, Th. W., Die Purpurbacterien und ihre Beziehungen zum Licht. Bot. Zeitg. 46: 661-669, 

 677-689,693-710,700-720. 1888. 



4 Gaidukov, N., Ueber den Einfluss farbigen Lichts auf die Farbung lebender Oscillarien. Abh. K. 

 Preuss. Akad. Wiss. Berlin, 1902. Anhang, Phys. Abh. V., p. 1-36. 



p But Molisch's studies indicate that the purple bacteria are not capable of the photo- 

 synthesis of carbohydrates from carbon dioxide and water. See: Molisch, Hans, Die Purpur- 

 bakterien nach neuen Untersuchungen, eine mikrobiologische Studie. 92 p. Jena, 1907. — Ed. 



