ASSIMILATION OF CARBON 



23 



light screens and the latter the prismatic spectrum. Both came to the con- 

 clusion that plants decompose carbon dioxide most readily under the influence 

 of the yellow light rays. Sachs^ divided the spectrum into two nearly equal 

 portions, by using a solution of potassium dichromate and one of ammoniacal 

 copper oxide, and found that decomposition of carbon dioxide proceeded almost 

 as energetically in the yellow portion of the spectrum as in direct sunlight, 

 while very little decomposition occurred in the blue-violet region. It is seen, 

 therefore, that it is not the so-called "chemical" rays that are needed for this 

 process, but chiefly the less refrangible rays of the first half of the spectrum. 

 Sachs determined the amount of oxygen given off, using the method of counting 

 gas bubbles (Fig. 2). C,\v\ 



The next problem was to discover in what rays of the sfirt half of the 

 spectrum the decomposition of carbonic acid was most 

 rapid. The most exact studies upon this point were 

 carried out by Timiriazev^ who arranged his experi- 

 ments as follows: Sunlight was reflected from a helio- 

 stat into a dark chamber and was then broken up by 

 a carbon bisulphide prism. Pieces of bamboo leaves 

 were enclosed in glass tubes, with air containing 5 per 

 cent, of carbon dioxide, and these tubes were placed in 

 various regions of the spectrum — in the red between 

 A and B, in the chlorophyll absorption band between 

 B and C, in the orange, in the yellow, and in the green. 

 At the conclusion of the experiment analyses of the 

 gas were made, by means of a very sensitive appa- 

 ratus capable of measuring extremely small amounts 

 of gas. Timiriazev's results are graphically repre- 

 sented in Fig. 13. The ends of the five ordinates, for 

 the five positions in the spectrum where the tubes were 

 exposed, are joined to form a curve, which represents 

 the relative rates of decomposition of carbon dioxide 

 in these different regions of the spectrum. The maxi- 

 mum decomposition occurs in the red, between B and C, in the region where light 

 is most strongly absorbed by chlorophyll. No decomposition occurs between A 

 and B (the line m represents the amount of carbon dioxide eliminated during 

 the experiment). These results were confirmed by Engelmann^ and Reinke.* 



'Sachs, J., Wirkungen farbigen Lichts auf Pflanzen. Bot. Zeitg. 22: 3S3-358, 361-367. 369-372. 

 1864. 



= Timiiiazev, K. A., (C.) On the assimilation of light by plants. [Russian.] St. Petersburg, 1875- 

 Timiriazeff, C, Recherches sur la decomposition de I'acide carbonique dans le spectre solaire, par les 

 parties vertes des v^gStaux. (Extrait d'un Ouvrage "Sur I'assimilatjon, de la lumiire par les v^g^taux," 

 St.-P6tersbourg, 187s; publiS en langue russe.) Ann. chim. et phys. F, 12: 355-396. 1877. 



' Engelmann, Th. W., Ueber Sauerstoflausscheidung von Pflanzenzellen im Mikrospectrum. Bot. 

 Zeitg. 40: 419-426. 1882. 



« [Reinke, J., Untersuchungen uber die Einwirkung des Lichtes auf die Sauerstoffausscheidung derPflan- 

 zen. II. Die Wirkung der einzelnen Strahlengattungen des Sonnenlichtes. Bot. Zeitg. 42 : 17-29,33-46. 

 49-59. 1884. See column 27. Idem, , Die Zerstorung von Chlorophyllosungen durch das Licht und eine 

 neue Methode zur Erzeugung des Normalspectrums. Ibid. 43: 65-70, 81-89, 97-ior, 113-117. 129-137. 

 1885. See column 84. Idem, Die Abhangigkeit des Ergriinens von der Wellenlange des Lichts. Sitz- 

 ungsber (Math.-Naturw. Mitth.), K. Preuss. Akad. Wiss. Berlin. 1893:301-314. 1893.] 



Pig. 13. — Graphs show- 

 ing relative rates of decom- 

 position of carbon dioxide in 

 different parts of the spec- 

 trum. (After Timiriazev.) 



