200 PHOTOSYNTHESIS 



quires such an elaborate mechanism, even in the lowest forms of plants, 

 that it is inconceivable that a photosynthetic process elaborating carbo- 

 hydrates existed without the intermediary of a Hving organism or in 

 very much simpler organisms than we now know. But there is no evi- 

 dence whatsoever that this was ever the case. So that speculations on 

 the origin of life must make assumptions which are so far removed from 

 any knowledge of which we are now in possession that the deductions 

 must of necessity have but a very limited significance. 



Virtually the only direct chemical evidence we have pertaining to the 

 limited number of hexoses which are found in nature are the experiments 

 of Nef on the effect of alkalies on the hexose sugars. In these investiga- 

 tions Nef found that equilibrium is reached with a mixture of these sugars 

 in an alkaline solution, that certain of the hexoses are never present in 

 such mixtures and that the amounts of those which are present show some 

 similarity to those found in nature. These experiments will be discussed 

 in the chapter on the Chemistry of Photosynthesis. 



h. Analysis of Hexoses. 



We can here but indicate some of the methods which have been used 

 for identifying and determining hexoses in plant material; for the de- 

 tails of the methods reference must be made to the literature cited. The 

 fermentation with baker's yeast is one of the most common tests for 

 hexose sugars. The sugars fermented are d-glucose, d-fructose and 

 d-mannose ; d-galactose is fermented much more slowly by ordinary yeast, 

 while with pure yeast pentoses and methylpentoses are not fermented. A 

 fermentation at 35° for 2-3 days thus accomplishes a separation of these 

 sugars. It is essential to make several blank tests to determine : 1, whether 

 the yeast with water alone produces carbon dioxide; 2, to determine 

 whether the yeast is active ; 3, whether the yeast affects pentoses. With 

 these points in view, a positive test points to d-glucose, d-fructose and 

 d-mannose; with the fermentation tubes 10 mg. of glucose can still be 



detected. 



When aldopentoses are treated with acids there is formed furfural 

 which has been used as a test to identify these sugars. Hexoses treated 

 in a similar manner, yield 4-oxymethylfurfural. The color reactions of 

 hexoses when treated with concentrated acids and phenols (e.g. Molisch 

 reaction) depend upon the formation of condensation products of the 

 phenols with the 4-oxymethylfurfural.^^ The latter compound is formed 

 more rapidly with ketoses than with aldoses, and on this fact is based a 

 distinctive reaction between aldoses and ketoses (Pinoff reaction). 



The 4-oxymethylfurfural in the presence of concentrated acid under- 

 goes a further change to formic acid and levulinic acid. The formation 

 of levulinic acid when hexoses are treated with 20 per cent hydrochloric 



"Nef, Ann. Chem., 376, 117 (1910). 



