306 PHOTOSYNTHESIS 



invariably supply the energy required for the reduction of carbon dioxide 

 and which lacks the power to bring about polymerization or decomposi- 

 tion of formaldehyde. The next experiment in this field must be made 

 with monochromatic ultra-violet light." 



The question may well be asked of what significance for the natural 

 process of photosynthesis are experiments on the action of ultra-violet light 

 on carbon dioxide and water? That the ultra-violet portion of the solar 

 spectrum plays only a very insignificant role or none at all in photo- 

 synthesis is almost certain, for the intensity of wave lengths shorter than 

 300 \i[i in the solar spectrum is very small and, moreover, photosyn- 

 thesis is carried on quite normally when the plants are behind glass which 

 is opaque to ultra-violet light. This is also substantiated by measurements 

 of the rate of photosynthesis with different wave lengths of light. On 

 the other hand the problem of photosynthesis is in a stage of develop- 

 ment in which any information relative to the action of light on the com- 

 ponents of the reaction may be of value. It is impossible to say from 

 which direction a solution will come. The danger lies in jumping at 

 conclusions, in hasty attempts to explain the whole phenomenon on the 

 basis of partial knowledge or of limited experimental observations. 



Photochemistry is still an undeveloped science. Chemists have not 

 been able to describe satisfactorily the mechanism of thermal reactions, 

 and photochemical reactions are apparently even more difficult to inter- 

 pret. The nature of fluorescence and the role which fluorescent sub- 

 stances may play in photochemical reactions is a field which will probably 

 have to be worked over before we can understand the chemistry of photo- 

 synthesis. This is suggested by the striking fluorescence of chlorophyll 

 and some of the pigments which are associated with it in the cell. It is 

 true that at present there is little information which could lead us to 

 believe that ultra-violet light is of any significance in relation to the 

 function of chlorophyll. In blue light chlorophyll fluoresces in the red, 

 about at 700 [i[i. Carbon dioxide and carbonic acid have no absorption 

 bands in the visible spectrum. Is it possible that in the photosynthetic 

 apparatus there can take place a "stepping up" of energy in the sense 

 that the absorption of a number of quanta in red light, for example, may 

 result in the reradiation of one quanta of ultra-violet? Have we in 

 photosynthesis a photochemical reaction taking place in a nonabsorbing 

 acceptor by reacting with a primary photo-acceptor? It is doubtful 

 whether we are as yet in possession of sufficient experimental data to 

 expect profitable speculation. 



b. The Reduction of Carbon Dioxide in the Presence o£ "Catalysts" 

 and Chlorophyll Preparations. 



There are some photochemical reactions proceeding easily in ultra- 

 violet light which require a catalyst in order that they may take place 

 in sunlight. Uranium and iron salts have been extensively used to bring 



