202 PHOTOSYNTHESIS 



in its breaking down and oxidation and the many syntheses in which glucose 

 finally becomes a component of a more complex molecule such as glucosides, 

 amines and proteins, cannot be followed here. 



While glucose has been obtained from a large variety of plants and 

 plant parts by means of extraction or through hydrolysis of condensation 

 products, the direct evidence of the existence of free glucose in leaves 

 is surprisingly meager. The presence of glucose is usually concluded from 

 the formation of glucose phenylosazone, the optical rotation or even only 

 from the fact that the leaf extract reduces Fehling's solution. In view of 

 the mixture of substances present in leaves any one of these criteria is 

 not sufficient to establish the presence of glucose. However, Deleano '' 

 has identified glucose and fructose in grape leaves by means of the methyl- 

 phenylhydrazone and the methylphenylosazone. 



An estimation of the amount of glucose in leaves has been obtained 

 usually by methods which employ a combination of the optical rotatory 

 power and reducing power of a mixture. As can be seen from the 

 analytical data which are given in a later portion of this chapter, the 

 amount of glucose in leaves varies greatly with the time of day and in 

 different plants. In general the amount of glucose runs from none to 

 about 3 per cent of the dry leaf material. 



Glucose is identified by means of the formation therefrom of saccharic 

 acid on oxidation with nitric acid. Saccharic acid forms characteristic 

 acid-potassium and silver salts.'** Glucuronic acid also forms saccharic 

 acid. For the identification of glucose are also valuable the following 

 compounds ; phenylosazone, p-nitrophenylhydrazone, (3-naphthylhydrazone, 

 p-bromphenylosazone and the p-nitrophenylosazone. Which of these com- 

 pounds is best suited for the identification of glucose depends upon what 

 other sugars are present in the mixture. Van der Haar "^^ has worked out 

 the best means of identifying various monosaccharides present in mixtures 

 by means of the different phenylhydrazine compounds. 



d-Mannose in the free state has been found in small amounts in plants. 

 There exists little information regarding the presence of mannose in 

 foliage leaves. The anhydride condensation products of mannose are 

 widely distributed as mannans."'' The alcohol mannitol is widely dis- 

 tributed in plants. It is a curious fact, reported by Knudson,«^ that 

 d-galactose and d-mannose are toxic to the roots of some plants. d-Man- 

 no*se forms a characteristic phenylhydrazone which is used for identifica- 

 tion as is also the p-bromphenylhydrazone and the p-tolylhydrazone. 



d-Galactose. The presence of this sugar in plants in the free form 

 has not been definitely established. It is, however, widely distributed as 



"Deleano, Zcit. physiol. Chem., 80, 79 (1912). 



^Gans and Tollens, Ann. chcni., 249, 217 (1888). , ^ ,■ a 



'•Van der Haar, "Anleitung zum Nachweis, zur Trennung und Bestimmung der 

 Monosaccharide und Aldehydsauren," 1920, p. 240. 



-Tsukamoto, Bull. Coll. Agri. Tokyo. 2, 406 (1897). Flatau and Labbe 5 J . 

 Soc. Chim. (3), 19, 408 (1898). Easterfield and Aston, Proc. Chem. Soc. 19, IVl 



(1903). 



•^Knudson, Am. Jour. Bot., 4, 430 (1917). 



