310 ASSIMILATION. 



825. Colored light and assimilation. Daubeny, in 1835, was 

 the first 1 to experiment systematically upon this subject. His 

 method 2 of investigation was as follows : "A certain number 

 of fresh leaves, which presented in each case an extent of surface 

 as nearly as possible equal, and had been previously ascertained 

 to give out equal quantities of oxygen, were introduced severally 

 into jars filled with water impregnated with carbonic acid gas, 

 placed on the surface of a pneumatic trough, and exposed for 

 a certain time to the influence of the solar rays. The jars in 

 which the leaves thus selected stood, were severally covered 

 over by a wooden screen which intercepted all light from the in- 

 cluded jar," excepting in front, where a frame was fitted, into which 

 (1) colored glass or (2) flat bottles filled with differently colored 

 liquids could be fastened, so that the light reaching the leaves 

 could be variously modified. The amount and character of the 

 gas escaping into the upper part of each jar were carefully 

 determined. The leaves used were those of Brassica oleracea, 

 Salicorma, Fucus, Tussilago, Cochlearia Armoracia, and Mentha 

 viridis. Besides plain glass, the following colored varieties were 

 employed : orange, red, blue, purple, green ; while the liquids 

 used were, for blue, ammonio-sulphate of copper, and for red, 

 port wine. 



In all cases Daubeny determined the amount of gas given off 

 by the leaves, and afterwards analyzed it in order to ascertain 

 the percentage of oxygen. He concluded from his experi- 

 ments, 8 " that the effect of light upon plants corresponds with 

 its illuminating rather than with its chemical, or its calorific 

 influence." 



826. J. W. Draper, in 1844, published an account of his ex- 

 periments upon the relations of green plants to light, as regards 

 the amount of assimilative activity indicated by the oxygen given 



1 Senebier and others had already conducted some inconclusive experiments 

 in nearly the same field. 



2 On the Action of Light upon Plants, and of Plants upon the Atmosphere 

 (Philosophical Transactions, 1836, p. 149). 



The activity of assimilation proper, as will be seen later, can be measured 

 with a very close approximation to accuracy, by the amount of oxygen gas 

 which is set free from the assimilating tissues, or, what amounts to substan- 

 tially the same thing, by the amount of carbonic acid decomposed by them. 

 For the sake of uniformity, the word assimilation is to be used in the follow- 

 ing paragraphs, even where the authorities cited refer to the process under 

 the terms decomposition of carbonic acid, evolution of oxygen, etc. The term 

 assimilation, in its restricted sense, was adopted by Sachs (1863). 

 8 Philosophical Transactions, 1836, p. 151. 



