ASSIMILATION OF CARBON 1 9 



As to the physics of the action of chlorophyll, it behaves as a sensitizer 1 and 

 renders the energy of the absorbed light effective in the decomposition of car- 

 bon dioxide. In an analogous manner the red light rays between lines В and С 

 of the spectrum rapidly decompose silver salts in the presence of chlorophyll, 

 although these salts are otherwise decomposed only by blue and violet rays. 



§4. Pigments Accompanying Chlorophyll. — Among the other pigments 

 accompanying chlorophyll, special attention should be given to carotin. 2 Boro- 

 din 3 was able to show that carotin (called erythrophyll by him) regularly ap- 

 peared in alcoholic leaf extract when he allowed this to form crystals under the 

 microscope. 



The chemical nature of carotin, and also some of the conditions of its forma- 

 tion in leaves, were first made clear by the investigations of Arnaud 4 and of 

 Willstätter and Mieg. 5 This pigment forms flat, rhombic crystals, which, with 

 one-sided illumination, appear blue-green on the illuminated side and orange- 

 red on the other. It is readily soluble in ether, chloroform and carbon bisul- 

 phide, less so in benzine, slightly soluble in hot alcohol, almost insoluble in cold 

 alcohol and insoluble in water. A carbon bisulphide solution of carotin is 

 blood-red; dissolved in concentrated sulphuric acid, carotin is bluish-violet. It 

 is a hydrocarbon, with the formula C 4 oH 56 , which is easily oxidized. It may be 

 transformed into Cholesterin. The carotin content of leaves varies with the 

 season of the year. A series of experiments continued throughout the summer 

 upon the leaves of stinging nettle and horse-chestnut showed that the carotin 

 content is greatest during the flowering season, for both plants. The formation 

 of carotin is also dependent upon light; green leaves of vetch contained 178.8 

 mg. of carotin, as compared to 34.0 mg. in the same quantity of etiolated leaves. 



It was shown by the work of Kohl 6 that carotin is widely distributed. It 

 is not limited to the green parts of plants but occurs also in flowers, fruits, seeds 

 and subterranean organs, and also in fungi. It may be extracted in large quan- 

 tities from carrots. 



The function of carotin is not yet clear, but its tendency to unite with oxygen 

 appears, at any rate, to be significant in connection with the photosynthetic 

 process, where reduction of compounds containing oxygen is known to occur. 



1 Tappeiner, H. von, Die photodynamische Erscheinung (Sensibilisierung durch fluoreszierende Stoffe). 

 Ergeb. Physiol. 8: 698-741. 1909. 



- Escher, Heinr. H., Zur Kenntnis des Carotins und des Lycopins. Zürich, 1909. 104 p. (Zürich Poly- 

 techn. Dissert. 1909-10.) [For a general discussion of the yellow pigments, see Haas and Hill, 1921. 

 (See note 3, p. 6.)] 



3 Borodin, J., Ueber krystallinische Nebenpigmente des Chlorophylls. Bull. Acad. Imp. Sei. St.- 

 Petersbourg 28: 328-350. 1883. 



4 Arnaud, A., Recherches sur les matieres, colorantes des feuilles; identite de la matiere rouge orange 

 avec la Carotine, C18H24O. Compt. rend. Paris 100: 751-753. 1885. Idem, Recherches sur la composi- 

 tion de la Carotine, sa fonction chimique et sa formule. Ibid 102:1110-1122. 1886. Idem, Sur la pres- 

 ence de la Cholesterine dans la carotte; recherches sur ce principe immediat. Ibid. 102 : 1310-1322. 1886. 

 Idem, Recherches sur la Carotine; son r61e physiologique probable dans la feuille. Ibid. 109: 911-914. 

 1889. 



5 Willstatter, Richard, and Mieg, Walter, Ueber die gelben Begleiter des Chlorophylls. Liebig's Ann. 

 Chcm. u. Pharm. 355: 1-28. 1907. 



6 Kohl, Friedrich Georg, Untersuchungen über das Karotin und seine physiologische Bedeutung in 

 der Pflanze. Leipzig, 1902. 



