62 THE SYNTHESIS OF CARBOHYDRATES 



and Pictet and Vogel * have synthesized sucrose from glucose 

 and fructose. 



SUMMARY OF THE EVIDENCE RELATING TO THE 

 FORMALDEHYDE HYPOTHESIS. 



It is obvious from this survey that the hypothesis of Baeyer 

 has influenced to a remarkable degree investigations on the 

 synthetic aspects of carbon assimilation ; this to a large extent 

 is psychological, for the hypothesis is attractive in that it is 

 plausible and lends itself readily to test-tube investigation. 

 The evidence readily centres around distinct nuclei — the 

 identification in the plant of the critical initial intermediate 

 products ; the effect of presenting the plant with some of 

 these initial substances in place of natural raw material ; and 

 the elaboration of the final from the supposed initial products 

 in the laboratory — and may be briefly considered under these 

 heads : — 



Carbon Monoxide. — -This gas, if produced, must be fixed 

 with extreme rapidity, for there is no evidence of its occur- 

 rence as such in assimilating tissues, which is remarkable for 

 a compound so inert. The evidence for its use by the plant 

 when substituted for carbon dioxide is inconclusive. 



Formaldehyde.— -There is no doubt that formaldehyde can 

 be obtained from green leaves, there being many compounds 

 which yield it more or less readily on decomposition. The 

 production of formaldehyde in artificial chlorophyll systems 

 was once considered strongly to support Baeyer's hypothesis, f 

 but the work of Jorgensen and Kidd, who were the first to use 

 pure chlorophyll in this connection, seriously discounts the 

 significance of the reactions since the formaldehyde has its 

 origin in the oxidation of the chlorophyll ; it is not produced 

 in such systems if oxygen be absent. Much of the evidence 

 relating to the production of formaldehyde from carbon dioxide 

 andwater under the influence of radiant energy is contradictory. 



Its balance strongly supports the view that free formalde- 



* Pictet and Vogel : " Compt. rend.," 1928, 186, 727. See Vol. I., 

 p. 113. 



f See, for example, Usher and Priestley : " Proc. Roy. Soc," B, 1906, 

 77»369; 1906,78,318; 1911,84,101. 



