296 



ASSIMILATION. 



The instructive similarit} 7 between the spectrum of the yellow 

 coloring-matter of chlorophyll and that of the so-called etiolin, 

 or yellow coloring-matter which can be extracted from blanched 

 leaves, is shown in the two figures here given. 



789. An alcoholic solution of chlorophyll undergoes very little 

 if any change when kept in the dark ; but even a short exposure 

 to strong light destroys its green color, and leaves the liquid 

 pale brown, or nearly colorless. When, however, strong sun- 

 light passes through a solution of chlorophyll before it reaches a 

 second receptacle filled with the same liquid, the first solution 

 protects the second for a considerable time ; and only after the 

 first has lost a portion of its green color can the second be also 

 acted upon. 



790. Sachs l has pointed out the interesting fact that green 

 leaves, especially those of delicate texture, become paler when 

 exposed to a very bright light, and resume their deep green 

 color when again subjected to a less intense light. If one leaf 

 is partially "shaded by another, the shaded leaf preserves its nor- 

 mal deep green color, while the leaf exposed to the light grows 

 distinctly paler. This effect, due probably to a change of posi- 

 tion of the chlorophyll grains, can be shown experimentally 

 in the following manner : Fasten closely to a green leaf, still 



1 Ber. iiber die Verhandlungen (Math. Phys. Classe) der Siichsischen Ge- 

 sellsch. xi., 1859, 226; and also in Experimental-physiologic, 186o. 



FIG. 151. The upper spectrum is that of the yellow constituent of chlorophyll from 

 Deutzia scabra; the lower, that of the coloring-matter of etiolated barley, in dilute 

 solution. (Kraus.) 



