258 PHOTOSYNTHESIS 



plish photosynthesis outside of the cell should therefore not seem sur- 

 prising even to those who do not believe that plasmatic or enzymatic ac- 

 tivity plays a role in the first steps. The difficulties confronting an extra- 

 cellular photosynthesis may, in fact, lie not so much in the first steps, the 

 reduction of the carbon dioxide, but in the removal of the reduction prod- 

 ucts, i.e., in transforming these into more stable compounds. An accumu- 

 lation of such intermediate products, even in concentrations which escape 

 our methods of analysis, may be sufficient to inhibit completely the course 

 of the reaction. Were this the case, in the normal course of the reaction, 

 we could expect a condition of steady state in which the concentration of 

 the intermediate products was such that it would escape our notice when 

 analytical methods are employed. That such conditions exist in the normal 

 photosynthetic process has been postulated in a number of theoretical 

 considerations of the problem. The treatment of a problem in this manner 

 may lead to valuable conclusions ; the danger lies in accepting the assump- 

 tion of the existence and nature of such intermediate substances, as proven. 



When the theories which have been advanced are studied critically, the 

 conclusion cannot be escaped that, up to the present time, none of these 

 theories has found substantiation on the basis of experimental facts. It 

 is true that substantiation has been repeatedly claimed for one theory or 

 another. But on account of the complex nature of the reactions which go 

 to make up the photosynthetic process, special care must be exercised to 

 avoid drawing conclusions from limited data, and more especially, gen- 

 eralizing from observations gained under limited or particular conditions. 

 A perusal of the publications which have appeared during the past 75 

 years also shows that the train of thought has been strongly influenced by 

 the type of investigation which was "in fashion" at the time the particular 

 theory was advanced. The entire subject of the chemical reactions con- 

 stituting the reduction of carbon dioxide, the liberation of oxygen and 

 the formation of carbohydrates is still in a purely hypothetical stage as 

 far as the explanation for this process is concerned. 



In the various attempts that have been made to describe the steps in- 

 volved in photosynthesis the course of reasoning has been very similar. 

 This has usually included one or more of the following forms : _ 



(1) A formulation of the theoretical steps from carbon dioxide and 



water to glucose. 



(2) Attempts to find in the plant substances which form the inter- 

 mediate steps demanded by the theory. 



(3) The feeding to the plant of these intermediate products with the 

 object of determining whether the plant is capable of using these sub- 

 stances for the synthesis of sugars or starch. 



(4) Attempts to effect a photochemical reduction of carbon dioxide 

 in zntro, including in some cases the use of chlorophyll preparations. 



In endeavoring to exercise critical judgment on the various theories 

 suggested to describe the chemical steps involved in the reduction of carbon 

 dioxide and water to sugar, it is well to bear in mind the old familiar 



