THE PRODUCTS OF PHOTOSYNTHESIS 203 



a component of galactosides and as an anhydride condensation product 

 with other sugars, as galactan, in gums and pectin. The alcohol dulcit 

 has also been found in plants. Galactose is identified by the formation of 

 the sparingly soluble mucic acid formed on oxidation with nitric acid.®^ 

 The o-tolylhydrazone and a-methylhydrazone are also used. 



d-Fructose has been found almost as widely distributed in plants as 

 glucose. It is found in the free condition in leaves and often in higher 

 concentration than glucose. As a component of sucrose it is especially 

 common as well as of tri- and polysaccharides such as rafifinose, melicitose, 

 inulin and levulan. The isolation and identification of fructose in mixtures 

 of other sugars, such as are obtained from plant material, are associated 

 with some difficulty. The Pinofif reaction is of value in detecting fructose 

 (also sorbose) in a mixture of aldoses and Neuberg ''^ recommends the 

 methylphenylosazone. The use of iodine and bromine to differentiate 

 between aldoses and ketoses has been used extensively of late and will be 

 described later in this chapter. 



d-Sarbose is the only other ketose which has been identified in plants 

 and this has been found in fruits and has not been reported as present in 

 leaves.^* 



i. Cycloses. 



A group of comjX)unds which is very widely distributed in plants, 

 though not found in large quantities, is derived from hexahydrobenzol. 

 These cyclic compounds have the same molecular formula as the hexoses, 

 CeHioOe, have a sweet taste, are soluble in water, exist in various stereo- 

 isomeric forms some of which are optically active, but do not reduce 

 cuproalkaline solutions. The inositols, C6H6(OH)6, are the most widely 

 distributed of these compounds. A phosphoric acid compound of i-inisitol 

 is the commonly occurring phytin. 



Very little is known regarding the mode of formation or function of 

 the cycloses in plants and little work has been done in this field. From 

 the viewpoint of photosynthesis the cycloses are of interest on account 

 of the theory of Crato who in 1892 suggested that inositol may be the 

 first product of the reduction of carbon dioxide in the photosynthetic 

 process. The ring is opened, yielding a straight chain aldohexose ( see 

 page 204). 



However, there has been no work done to test the theory ; there are 

 no quantitative data available which would throw light on the inositols 

 as direct products of photosynthesis. Crato's ^^ theory has in its essentials 

 been restated by Kogel. 



"'Van der Haar, Biochcm. Zcit.. 81, 263 (1917). Braun, Bcr. chem. Ges., 49, 

 1266 (1916). 



"Neuberg, Ber. chctn. Gcs., 35, 959 (1902). 



"Dobner, Ber. chem. Gcs., 27, 345 (1894). 



"Crato, Ber. bot. Ges., 10, 250 (1892). Kogel, Biochcm. Zcit., 95, 313 (1919) ; 

 97, 21 (1919). For inositols see Armstrong, "Carbohydrates," etc., 1924, p. 112. 

 Maquenne, "Les Sucres," 1900, p. 189. Czapek, "Biochem. der Pflanzcn," III, 1921, 

 p. 480. Lippmann, "Chem. der Zuckerarten," I, 1904, p. 1007. 



