178 LIGHT, VEGETATION AND CHLOROPHYLL 



On the other hand, if the quantity of chlorophyll is reduced, 

 the fixation of energy will be affected and it will be useless to 

 lengthen the intervals between the flashes because the efficiency 

 will not be increased. A decrease in the quantity of carbon 

 dioxide, or the presence of a narcotic, has a similar eff'ect on 

 the luminous phase and on that only. 



The Luminous Phase 



In 1901, Friedel observed that pulverized green leaves give 

 off" a Uttle oxygen when they are illuminated. Hill discovered in 

 1937 that this liberation of oxygen is prolonged if ferric salts, 

 which become ferrous, are added to this green powder; while 

 the oxygen is being given off", a reduction is accompUshed by 

 Hght through the agency of chlorophyll. 



This experiment is made by extracting the chloroplasts and 

 suspending them in an artificial medium. Not all leaves can 

 be used satisfactorily; those from trees are rather dis- 

 appointing, but lettuce or spinach provides excellent material. 

 The pulverization must not be carried too far, for if the 

 grains of chlorophyll are spUt up and reduced to too small a 

 size their activity soon decUnes. 



Although the chloroplasts are slightly modified by being 

 outside the cell, investigations have shown that this reaction 

 obeys the same laws as photosynthesis and is modified 

 similarly by the same circumstances. The Hill reaction there- 

 fore fairly accurately represents the luminous phase of photo- 

 synthesis, which can now be studied in vitro. 



The energy fixed by chlorophyll can be estimated only to 

 the extent that it is seen to be used. Since photosynthesis uses 

 it to Uberate electrons, i.e., to reduce, a substance capable of 

 reduction is associated with the chloroplasts. 



Hill used ferric oxalate which is transformed into ferrous 

 oxalate by means of the electrons Uberated by the release 

 of oxygen. To enable this reaction to continue, he added 

 ferricyanide which reoxidizes the ferrous oxalate, and the 

 reduction recommenced. In view of the potential diff'erence 

 (1-2 to 1*3 volt) separating carbon dioxide from the 



