298 PHOTOSYNTHESIS 



that the plant is capable of forming starch from a variety of substances. 

 The formation of starch in itself is probably not a very good criterion to 

 follow in endeavoring to judge as to possible intermediate products of 

 photosynthesis. Starch formation occurs only when the sugar concen- 

 tration has attained a sufficiently high value, and the enzymes which are 

 active in starch synthesis are affected by a wide variety of substances and 

 conditions which do not influence photosynthesis. 



If, finally, we ask how much have these feeding experiments con- 

 tributed to establishing or disproving the formaldehyde theory of photo- 

 synthesis, we are forced to the conclusion that it has been very little. 

 There are other substances from which the plant can also produce starch 

 and much more easily than from formaldehyde. Here it must be re- 

 membered also that the attempts to establish the value of formaldehyde 

 as a substance which the plant can use have been more numerous and 

 persistent than with any other substance. The first results with formalde- 

 hyde were also negative. Doubtless there are other substances, which if 

 studied with the care used with formaldehyde, would also yield positive re- 

 sults. Probably as much as can safely be concluded is that these feed- 

 ing experiments indicate that formaldehyde as a possible intermediate 

 product need not be excluded. Accepting the positive results, these still 

 do not prove that formaldehyde is the intermediate product in the re- 

 duction of carbon dioxide and water to carbohydrates. It should be 

 mentioned here that the ability to form carbohydrates from formalde- 

 hyde is not confined to the chlorophyllous cells ; the liver of some ani- 

 mals can convert formaldehyde to glycogen.^* A definite proof of the 

 formaldehyde theory is certainly not afforded by any of the feeding 

 experiments. 



The same may be said of the feeding experiments with organic acids, 

 although their tolerance by the plant is notably higher than that of 

 formaldehyde, and organic acids have long been recognized as components 

 of plants, though their value as starch producing substances is largely 

 confined to algae and very little is known of the manner in which this 

 is accomplished. The feeding exi>eriments with organic acids also give 

 little support to the theory of Liebig. 



4. The Reduction of Carbon Dioxide 



a. The Direct Reduction of Carbon Dioxide. 



Many different means have been employed to effect a reduction of 

 carbon dioxide. While most of these are very different from any reaction 

 that could take place in a living plant, and have therefore little direct 

 bearing on the photosynthesis problem, they, nevertheless, are of inter- 

 est in this connection and have thrown considerable light on the be- 

 havior of carbon dioxide. From the chemical viewpoint and especially 



•"Grube, Pflilger's Arch., 121, 636 (1908). 



