196 PHOTOSYNTHESIS 



stance the change of gkicose into fructose and vice versa, this is not 

 of the nature of the conversions assumed. The hexose sugars found in 

 plants are all of the dextro series. We have no evidence that the optical 

 antipodes of these, the levo series, are ever formed in the plant. At 

 the same time, it is perhaps weW to bear in mind that there have been 

 very few^ thorough researches directed definitely to establish the absence 

 of the levo sugars. With the great mixture of substances existing in 

 leaves and the difficulties entailed in separating these, such criteria as 

 the optical rotation may not serve adequately to detect small amounts 

 of the levo compounds. There is little reason for believing that the 

 levo series v^ould have a greater rate of activity than the dextro. On 

 the other hand, as is mentioned below, catalysts which are themselves 

 asymmetric favor reaction with one of two optical antipodes. Also, it 

 is a rather common phenomenon that living organisms are capable of 

 using only one of two optical antipodes thus resulting in the resolving 

 of d-1 compounds, a fact which dates to the classical researches of 

 Pasteur. Similar properties have been found characteristic of a num- 

 ber of enzymes. Fischer first assumed the formation of an inactive 

 racemic sugar; this was split by the action of the organism, resulting 

 in the one active component remaining. This view, which Fischer ^' 

 abandoned later, has been elaborated to some extent by others. But this 

 is purely speculative, for as far as the evidence goes, we cannot accept 

 the existence of even small quantities of the levo sugars nor of the other 

 hexoses such as gulose, idose, talose, etc. 



It is interesting that while of the hexoses only members of the 

 dextro series have been identified, viz. d-glucose, d-mannose, d-fructose 

 and d-galactose, both dextro and levo pentoses have been reported as 

 components of plants. The l-araliinose has been found in the leaves of 

 Adonis venialis by van Ekenstein and Blanksma ^^ and is also a compo- 

 nent of gums such as gum arable and cherry gum. rf-Arabinose has been 

 reported as a component of the glucoside barbalin.^'* This is one of 

 the few instances of the occurrence of lioth the d an 1 modification of a 

 carbohydrate in plants.*" 



2. A second possibility which may account for the formation of 

 optically active products in the photosynthetic process are asymmetric 



"'Fischer, Ber. chem. Ges.. 23, 394, 2138 (1890). Winther, ibid., 28, 3022 

 (1895). Werner, "Lehrbuch der Stereochemie," Jena. 1904, p. 63. Pfeffer, Jahrb. 

 wiss. Bot., 28, 220 (1895). 



"%an Ekenstein and Blanksma. Chem. Zentr. (1908), I, 119. Bauer, Jour, prakt. 

 Chem (2). 34. 47 (1886). Kiliani, Ber. chem. Ges., 19, 3030 (1886). Tollens, 

 Landw. Vers., 39, 425 (1891). 



=«Leger, Compt. rend., 134, 1111, 1584 (1902); 150, 983, 1695 (1910); 155, 172 



(1912). 



*" Mention should also be made of the occurrence of the inactive form of galac- 

 tose. Winterstein, Ber. chem. Ges.. 31, 1571 (1898) reports i-galactose together with 

 d-galactose as a product of the hydrolysis of chagual gum, and Tollens and Oshima, 

 Ber. chem. Ges., 34, 1422 (1901), report obtaining i-galactose from the Japanese 

 nori. The reported 1-glucose in Grindelia by Power and Tutin, Proc. Avier. Pharm. 

 Ass., 1905, p. 5, was in all probability d-fructose. 



