4 H. A. SPOEHR 



branches of science, which are ordinarily but remotely con- 

 nected with plant physiology. Although some of these studies 

 have proved to have little application to the problem under con- 

 sideration, there are, nevertheless, many results which may prove 

 of great value. The three divisions referred to are: 



1. The reduction of carbon dioxide to formaldehyde. 



2. The detection of formaldehyde in illuminated green plants. 



3. Experiments in which plants were given formaldehyde as 

 the only source of carbon. 



It may be of interest to review briefly some of the experiments 

 which have been made in these three fields, and to discuss their 

 physiological bearing. 



A great variety of methods have been tried to reduce carbon 

 dioxide. Ballo,^ in 1884, got formic acid from carbonic acid by 

 treating the aqueous solution with sodium or potassium. The 

 attempts of Lieben^ to ascertain the first reduction product of 

 carbonic acid by the action of various reducing amalgams and 

 nascent hydrogen, resulted in the formation of formic acid ex- 

 clusively. In the various attempts that were made to reduce 

 carbonic acid and its salts by electrolysis, formic acid was always 

 the only product. Coehn and Jahn'' also call attention to the 

 interesting fact that the normal carbonates, and H2CO3 in the 

 presence of acids, were not reduced, only the bicarbonates and 

 the aqueous solution of CO 2 could be reduced. From this they 

 conclude that neither the CO3 ion nor the undissociated H2CO3 

 was reduced, but the HCO3 ion. By the use of metallic magne- 

 sium Fenton^ finally succeeded in reducing carbonic acid to for- 

 maldehyde, with formic acid as a probably intermediate step ; he 

 also showed that weak alkalies seem to aid materially in this 

 reaction. 



^ Ballo, W., Ein Beitrag zur Pflanzenchemie. Ber. deut. Chem. Ges. 17:6- 

 12, 1883. 



* Lieben, H., Ueber Reduktion der Kohlensaure bei gewdnlicher Temperatur. 

 Monatsch. f. Chem. 16: 211-47, 1895; 18: 582-88, 1897. 



' Coehn und Jahn, Ueber elektrolytische Reduktion der Kohlensaure. Ber. 

 deut. Chem. Ges. 37:2836-48, 1904. 



* Fenton, H. J. H., The Reduction of COo to Formaldehyde in Water Solu- 

 tion. Jour. Chem. Soc. 91:687-94, 1907. 



