SEARCH FOR FORMALDEHYDE IN LEAVES 255 



out in nitrogen or carbon dioxide, hexenaldehyde also was absent; it can 

 thus be considered as an oxidation product. The material from which it 

 is formed is unknown; but it must be even more strongly reduced than 

 hexenaldehyde itself, and thus even less likely to be an intermediate of 

 photosynthesis. 



C. The Formaldehyde Problem* 

 1. The Search for Formaldehyde in Leaves 



Because of the popularity of Baeyer's "formaldehyde theory" (1870), 

 no other compound has been so eagerly searched for in plants as formal- 

 dehyde — and with such uncertain results. Categorical statements that 

 formaldehyde does occur in leaves have been answered by no less cate- 

 gorical denials. Since formaldehyde is poisonous to plants, nobody had 

 ever expected to find a large quantity of this compound in leaves. It was 

 therefore necessary either to apply very sensitive methods of assay, or to 

 "trap" formaldeh3^de by a reagent which could be left in the cells for a 

 certain time without disturbing photosynthesis. 



Earlier investigations were made by the direct analytical method. 

 When Reinke (1881) discovered the presence of an aldehyde in the 

 products of steam distillation of leaves (cf. page 252), he at first thought 

 it to be formaldehyde; but Curtius and Reinke (1897) showed that it 

 lacked the specific properties of this compound. Pollacci (1899^- ^ 1907) 

 obtained positive formaldehyde tests with distillates of green leaves; 

 but his results were contested by Plancher and Ravenna (1904). Grafe 

 (1906) also claimed positive results, with a new reagent, diphenylamine 

 and sulfuric acid; but Curtius and Franzen denied that it gives a color 

 reaction with formaldehyde at all. Curtius and Franzen (1912) ob- 

 tained formic acid by the oxidation of the aldehj^de fraction of leaf 

 distillates, and considered this as an indirect proof of the presence of 

 formaldehyde; but Fincke criticized this conclusion and Curtius and 

 Franzen (1915) found later that oxidation by silver oxide can also pro- 

 duce formic acid from methanol (which is present in leaf distillates). 

 Fincke (1913) used a new reagent (fuchsinsulfuric acid in the presence 

 of hydrochloric acid), and concluded that the formaldehyde content of 

 illuminated leaves is less than 5 X 10~^%. He found further that, if 

 formaldehyde is supplied to the leaves from outside, it is not found in the 

 analysis, but is destroj^ed by the plant cells. 



Schryver (1910) claimed to have identified formaldehyde in chloro- 

 phyll preparations, extracted from leaves by alcohol, evaporated and 

 again extracted with ether, by means of Rimini's reagent (phenyl- 

 hydrazine hydrochloride, potassium ferricyanide, and hydrochloric acid) . 



* Bibliography, page 275. 



