THE CHEMISTRY OF PHOTOSYNTHESIS 259 



warning of Pfefifer. This runs to the effect that it is a very confusing 

 error to presume that an organism must, in its metabolic economy, follow 

 a course which seems to man. under the influence of existing chemical 

 and physical knowledge, the nit)st plausible course. All of these theories 

 are still dealing with probabilities drawn largely from experiences gained 

 from laboratory experiments carried out in vitro. How far one is justified 

 in applying such experiences to reactions taking place in the living or- 

 ganism, physiological experimentation only can determine. 



Regarding the second mode of formulating theories of the photo- 

 synthetic process, that of discovering in the plant the substances which 

 the theory demands are intermediate products in the process, it must be 

 borne in mind that the number of compounds found in plants is very great 

 and many different types are represented. It is very difficult to associate 

 directly the presence of any compound in a green leaf with the photo- 

 synthetic process. Thus, for example, the presence of formaldehyde in 

 illuminated leaves is evidence of only limited value in favor of the theory 

 that formaldehyde is an intermediate product in the reduction of carbon 

 dioxide to sugars, for formaldehyde is formed in the photolysis of a number 

 of substances which are almost constant components of leaves, so that the 

 presence of formaldehyde may be ascribed to more than one cause. More- 

 over, the concentration of highly reactive intermediate products is probably 

 very low and such products would consequently be very difficult to detect. 



The third argument, that of drawing conclusions regarding inter- 

 mediate products from the fact that the feeding to plants of such products 

 results in the synthesis of sugar or starch, must also be applied with great 

 caution. Experience has taught us that plants are capable of forming 

 starch from a large variety of compounds some of which have never been 

 found as normal components of leaves. 



Finally, the fourth argument, that of drawing conclusions regarding 

 the photochemical reduction of carbon dioxide from in zntro experiments 

 will be dealt with at greater length where these reactions are discussed more 

 in detail. 



It should perhaps be emphasized that some of the schemes which have 

 been proposed to describe the chemical steps in the conversion of carbon 

 dioxide to carbohydrates cannot be dignified with the term theory, though 

 they have been spoken of as such. In some cases they are mentioned quite 

 "en passant." as mere suggestions in the discussions of subjects which have 

 little application to the problem of photosynthesis. 



1. Theories Regarding the Reduction of Carbon Dioxide and 



Water to Carbohydrates 



a. Organic Acids. 



Probably the first attempt to picture the course of the reaction leading 

 from carbon dioxide and water to starch was made by Liebig.^ He re- 



'Liebig, Ann. Chem., 46, 62 (1843). 



