ASSIMILATION OF CARBON 



15 



ployed double-walled bell-jars with colored liquids, as light screens for isolating 

 certain regions of the spectrum (Fig. 8) . Solutions of potassium dichrdmate and 

 of ammoniacal copper oxide [copper sulphate solution to which an excess of 

 ammonia water is added] were most frequently used; the first, in medium con- 

 centration, permits the passage of the rays of the less refrangible half of the 

 spectrum (red, orange, yellow and a part of the green), while the second trans- 

 mits the remainder of the visible rays (the rest of the green and all of the blue 

 and violet). Thus, by the use of these liquids, the spectrum is separated into 

 two parts. [Of course the intensity of the light transmitted is considerably 

 decreased.] 



In weak light plants become green sooner under the yellow solution, but in 

 strong light more quickly under the blue. This may be explained by supposing 

 that in weak light the formation of chlorophyll occurs almost exclusively, under 

 the influence of the less refrangible rays, which are most favorable, while in 

 strong light, besides chlorophyll formation, an active de- 

 composition also takes place. Experiments upon the de- 

 composition of alcoholic solutions of chlorophyll under 

 colored bell-jars have shown that this process is especially 

 pronounced in the less refrangible half of the spectrum; 

 greening in plants is thus seen to be weaker in strong yellow- 

 red light because a very rapid destruction here accompa- 

 nies the formation of chlorophyll. But another explana- 

 tion is also possible: strong light may not act directly upon 

 chlorophyll that has already been formed but may, some- 

 how, have a harmful effect upon some process antecedent 

 to chlorophyll formation; this might explain why less 

 chlorophyll accumulates in strong light. 



Plants do not become green under the non-luminous 

 heat rays. In order to separate this portion of the spec- 

 trum, Tyndall's solution is used, iodine in carbon bisul- 

 phide; in low concentrations the rays between Fraunhofer lines A and B are 

 transmitted, but these, produce no green color. In ultra-violet light green- 

 ing is very slight. 



The production of chlorophyll is dependent upon temperature. Medium 

 temperatures are most favorable, and no greening occurs at very low or at very 

 high temperatures. Wiesner obtained the following results from experiments 

 with etiolated barley seedlings. 



Temperature ' Time Required 



FOR Greening 



Z)cg.(^ •. Hours 



2-4 * (No greening) 



4- S 7-25 



10 , 3 -SO 



18-19 '-^7 



30 i-S8 



37-38 4-00 



40 (No greening) 



Fig. 8.— Double- 

 walled bell-iar with 

 colored solution filling 

 the space between 

 the walls. 



