26 PHYSIOLOGY OF NUTRITION 



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. 1 In the Cyanophyceae the maxumim 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, 2 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 3 upon the influence of colored light upon 

 the color of Oscillaria. This alga tends to assume the color complementary to 

 that of the light acting upon it, and the longer the organism remains in the 

 colored light the more pronounced is the response. The following kinds of 

 illumination produced the following colorations in the organism. 



Color of Light Color of Alga 



Red Green 



Brownish-yellow Blue-green 



Green Reddish 



Blue Brownish-yellow 



The principle illustrated by this phenomenon was designated by Gaidukov as 

 the law of complementary chromatic adaptation. 



The amount of light 6 necessary for the decomposition of carbon dioxide is 



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



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

 677-689, 693-710, 709-720. 1888. 



3 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. 



* Kreusler, U., Ueber eine Methode zur Beotachtung der Assimilation und Athmung der Pflanzen und 

 iiber einige diese Vorgange beeinflussende Momente. Landw. Jahrb. 14: 913-065. 1885. Timriazeff, 

 C, Sur le rapport entre l'intensit6 des radiations solaires et la decomposition de l'acide carbonique par les 

 vegetaux. Compt. rend. Paris 109: 370-382. 1889. Pantanelli, Enrico, Abhangigkeit der Sauerstoff- 

 ausscheidung belichteter Pflanzen von ausseren Bedingungen. Jahrb. wiss. Bot. 39: 167-228. 1904. 

 Lubimenko, W., Sur la sensibility de 1' appareil chlorophyllien des plantes ombrophiles et ombrophobes. 

 Rev. g6n. Bot. 17: 381-415. 1915. Idem, concentration du pigment vert et l'assimilation chlorophyl- 

 lienne. Ibid. 20: 162-177, 217-238, 253-267; 285-297. 1908. Idem, Production de la substance seche 

 et de la chlorophylle chez les vegfitaux superieurs aux differentes intensites lumineuses. Ann. sci. nat. 

 Bot. IX, 7: 321-415. 1908. 



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. ]ena, 1907. 

 (A misstatement occurred here in the first printing.) — Ed. 



