ASSIMILATION OF CARBON 2$ 



region of the red to that of the yellow-green, according to the hour of the day. 

 Finally, chemically active or "actinic" rays, with a maximum in the violet 

 region, are frequently differentiated. The term actinic rays really refers to 

 the power of light to decompose silver salts, which is most pronounced in the 

 blue-violet region of the solar spectrum. Many other compounds are decom- 

 posed by light, however, frequently in other regions than the blue-violet, and 

 the wave-lengths producing such decomposition are those that are absorbed 

 by the substances decomposed: thus, chlorophyll is most rapidly decomposed 

 by rays between B and C, exactly the ones most completely absorbed by 

 chlorophyll. Therefore, the curve of chemical intensity, as usually given, has 

 no importance excepting with reference to silver salts: there are no specific 

 "chemical" rays. 



Researches upon the influence of light on the decomposition of carbon di- 

 oxide and water by plants fall into two groups. One group includes studies 

 dealing with the qualitative side of the question, as to which rays or wave- 

 lengths are most effective in the process. The other includes quantitative in- 

 vestigations, as to how much energy is needed for this decomposition. The 

 first qualitative work was done by Daubeny'" and Draper" the former using 

 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 1 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). 



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

 spectrum the decomposition of carbonic acid was most rapid. The most exact 

 studies upon this point were carried out by Timiriazev, 2 who arranged his 

 experiments as follows: Sunlight was reflected from a heliostat 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 



1 Sachs, J., Wirkungen farbigen Lichts auf Pflanzen. Bot. Zeitg. 22 : 353-358, 361-367, 369-372. 1864. 



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

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

 vertes des vegetaux. (Extrait d'un Ouvrage "Sur l'assimilation, de la lumiere par les v6getaux," St.-P6ters- 

 bourg, 1875; publie en langue russe.) Ann. chim. et phys. V, 12 : 355-396. 1877. 



'" Daubeny, Charles, On the action of light upon plants, and of plants upon the atmosphere. 

 Phil, trans. Roy. Soc. London 126: 149-175. 1836. — Ed. 



n Draper, John W., On the decomposition of carbonic acid gas and the alkaline carbonates 

 by the light of the sun. Phil. mag. 777, 23: 161-175. 1843. Idem, Scientific memoires. 

 473 P- New York, 1878. P. 184-185.— Ed. 



