166 THE CARBOHYDRATES 



action of dilute caustic soda on a substance called glycerose, 

 which is obtained by the oxidation of glycerol. Similarly 

 arabino-ketose, a pentose which has not yet been identified in 

 plants, has been synthesized by the action of calcium carbon- 

 ate, a very mild catalysing agent, from formaldehyde. From 

 the acrose thus obtained, Fischer was able by an elaborate 

 series of reactions to prepare ordinary fructose or levulose. 



It is thus seen that the conversion of formaldehyde to 

 fructose is reasonably complete, and when it has been proven 

 beyond all shadow of doubt that formaldehyde can be formed 

 from carbon dioxide and water by a photosynthetic method, 

 the chain of chemical evidence will be complete. In order to 

 bring the vital process into line with the laboratory process, it 

 is necessary to prove the presence of the intermediate pro- 

 ducts in the chlorenchyma of plants, together with suitable 

 catalysts. 



With regard to the formation of the higher carbohydrates 

 from the fructose, practically nothing is known, and unfortu- 

 nately our present knowledge of the constitution of disac- 

 charides is limited. 



While it is a very easy matter to determine what two 

 sugars are obtained by the hydrolysis of any given disaccharide, 

 it is as yet merely a matter of speculation as to how these two 

 sugars ure united in the disaccharide molecule. In the case 

 of cane sugar, for example, it is known that it yields on hydro- 

 lysis a mixture of dextrose and levulose, and in attempting to 

 assign a structural formula to cane sugar it must be borne in 

 mind that the substance has no longer the properties of either 

 an aldehyde or a ketone,* and therefore the formula must be 

 without the characteristic CHO or CO groups. 



The formation of dextrose does not present so great a 

 difficulty, for fructose has been converted into dextrose, in 

 vitro; it is only necessary to find a suitable agent in the 

 green tissues of plants to bring about the isomerization. 

 Thus, having the dextrose and fructose, the sucrose could be 

 produced by their condensation. But at the same time it 

 must be remarked that the synthesis of disaccharides from 

 monosaccharides has been achieved only in a very few cases, 



* Cane sugar does not react with phenylhydrazine, and does not reduce 

 Fehling's solution. 



