THE MECHANISM OF PHOTOSYNTHESIS 33i 



the entire complex of chemical, enzymatic, and photochemical reactions in- 

 volved in photosynthesis can occur in dried leaves as well as in living cells. 



4. The Formaldehyde Theory. — In 1870 the German chemist, Baeyer, 

 published an hypothesis of photosynthesis which has stimulated study and 

 investigation ever since. Baeyer suggested that the carbon dioxide of the 

 air combined with water forming formaldehyde (CHoO) with the elimination 

 of oxygen (Oo). This hypothesis is still favorably regarded by many investi- 

 gators and many attempts have been made to test its validity. Three prin- 

 cipal lines of investigation have been followed : ( i ) attempts to detect formal- 

 dehyde in living green cells during photosynthesis, (2) attempts to demon- 

 strate the synthesis of carbohydrates in plants immersed in dilute solutions of 

 formaldehyde, and (3) attempts to duplicate the synthesis of formaldehyde 

 from carbon dioxide and water in vitro. 



Klein and Werner (1926) tested the leaves of a number of species of 

 plants during active photosynthesis for formaldehyde with dimedon, a com- 

 pound which rapidly combines with formaldehyde, yielding readily identifiable 

 crystals of the compound formaldomedon. Positive tests were obtained for 

 formaldehyde which was present only in very small quantities (0.008-0.015 

 g. per 10 g. fresh leaves). This indicates that if formaldehyde is an inter- 

 mediate product of photosynthesis it must be very rapidly condensed to sugars. 

 In such low concentrations formaldehyde probably would have no toxic effect 

 upon living cells. Klein and Werner reported that they could obtain no tests 

 for formaldehyde in leaves in the dark, in leaves deprived of carbon dioxide, 

 in macerated leaves, in chlorophyll extracts, or in killed or narcotized leaves. 

 Barton-Wright and Pratt (1930), however, claim that formaldomedon can 

 be produced by a photochemical reaction between carbonic acid and dimedon. 

 Their results are in conflict with those of Klein and Werner, and further 

 work is needed before the results of the latter investigators can be accepted 

 without question. 



The results of a number of investigators have seemed to show that green 

 plants can use formaldehyde in very low concentrations in the synthesis of 

 sugars. In a recent critical investigation Paechnatz (i937). however, was 

 unable to confirm these results. Not only was there no evidence that any 

 of the several species investigated utilized formaldehyde in photosynthesis, but 

 this compound was found to be highly toxic in very dilute concentrations. 

 With Elodea canadensis, for example, toxic effects were noted as soon as the 

 proportion of formaldehyde in the solution exceeded about 0.0004 per cent. 



That formaldehyde can be polymerized into sugars in the laboratory by 

 certain treatments has been known for many years. In recent years Baly 

 and his coworkers (Baly and Davies, 1927; Baly and Hood, 1929) have 



