56 THE SYNTHESIS OF CARBOHYDRATES 



of certain external conditions such as low temperature.* The 

 problem has been considered by Meyer f who describes the 

 presence of a colourless fluid of unknown composition and 

 having the physical properties of an oil which occurs in the 

 chloroplasts of Tropceolum. This substance, which is not a 

 fat, since it gives on distillation hexylenealdehyde, increases 

 with assimilation, and is considered by Meyer to be a definite 

 product of carbon assimilation. 



HYPOTHESES CONCERNING THE SYNTHESIS OF CARBO- 

 HYDRATE BY THE GREEN PLANT. 



With the problems associated with the march of events 

 from the initial carbon dioxide and water to the final carbo- 

 hydrate, the realms of theory are entered. The hypotheses 

 are many and have this in common — none entirely satisfy the 

 plant physiologist ; for which reason it is proposed to consider 

 but few in any detail on the present occasion, not that those 

 discarded are barren of ideas— indeed, some contain valuable 

 suggestions — but that their examination, more especially 

 when unsupported by experimental evidence, would tend rather 

 to obscure than to clarify the problem. 



In 1870 Baeyer put forward the hypothesis that the carbon 

 dioxide is split up by the plant into carbon monoxide and 

 oxygen, and that the water is concurrently resolved into its 

 constituent elements. The carbon monoxide and hydrogen 

 thus produced then combine to produce formaldehyde, which 

 undergoes polymerization, and so forms a hexose. 



These changes may be represented in the following equa- 

 tions : — 



/i. C0 2 = CO + O 

 U. H a O = H 2 + O 



3. CO + H 2 = CH 2 



4. 6(CH 2 0) = C 6 H 12 6 



Thus, according to the theory, there are two distinct actions, 

 the first leading to the formation of formaldehyde, and the 

 second to the production of sugar. 



* See Vol. I. 



t Meyer: " Ber. deut. bot. Gesells.," 1917, 35> 5 8( J ; 1918, 36, 235, 

 674. 



