260 INTERMEDIATES IN REDUCTION OF CO2 CHAP. 10 



green leaves. One could discuss the weight of this fact as an argument 

 in favor of Baeyer's hypothesis (c/. page 247) ; but the fact itself seemed 

 to be established beyond doubt. Recently, however, the correctness of 

 these results was challenged by Paechnatz (1938) in Noack's laboratory. 

 She experimented with Elodea, Chlorella and Tropaeolum, and found no 

 evidence whatsoever of the capacity of these plants to utilize form- 

 aldehyde for the synthesis of sugars. Even with concentrations as low 

 as 0.003% (that is, considerably below Sabalitschka's optimum), she 

 observed nothing but poisoning, both of respiration and of photosynthesis. 

 The only positive effect was an increase in the relative quantity of sugars 

 in the dry substance of formaldehyde-fed plants. However, this differ- 

 ence was caused by the fact that sugars were less affected by ex-osmotic 

 processes caused by formaldehyde poisoning than other constituents of 

 the cells. The absolute quantity of sugars was decreased rather than 

 increased by formaldehyde "feeding." Attempts to repeat Bodnar's ex- 

 periments on polymerization of formaldehyde by leaf powder also fell 

 short of positive results. Formaldehyde was found to disappear in the 

 presence of plant cells — as observed earUer by Fincke (1913) — but this 

 was caused by catalytic oxidation rather than by polymerization. 



Thus, the problem of formaldehyde assimilation by green plants 

 remains open. Paechnatz' suggestion of errors which might have marred 

 the results of earlier authors does not account for all of their positive 

 experiments, especially those in which the "formaldehyde leaves" were 

 found to possess a higher dry weight and a higher sugar content than 

 fresh leaves. 



Bottomley and Jackson (1903) asserted that Tropaeolum majus can grow if carbon 

 monoxide is supplied to it instead of carbon dioxide, and saw in this result a confirmation 

 of Baeyer's theory. Their observations have never been repeated, and one may venture 

 to suggest that the measures taken to prevent the access of small quantities of carbon 

 dioxide to the plants were not as efficient as the authors thought them to be. 



3. Feeding of Plants with Other Low Molecular Weight Compounds 



While the acceptance of formaldehyde as food by algae and other 

 green plants remains a subject of controversy, doubts also arise as to 

 whether even unquestionably positive results of formaldehyde feeding 

 would carry much weight as arguments in favor of Baeyer's theory. 

 These doubts derive from the fact that not only hexoses and pentoses (c/. 

 Chapter 3) and their close derivatives (sugar alcohols, uronic acids, etc.), 

 but also compounds with shorter carbon chains (C2 to Cb) can be utiHzed 

 by plants for conversion into starch in the dark. Thus, Meyer (1885) 

 found that leaves can synthesize sugar from glycerol, and Bokorny (1897) 

 gave a long list of compounds which algae can utiHze for the production 



