28 PROXIMATE CONSTITUENTS OF PLANTS 



the plant gets older the digestibility decreases until finally the 

 cellulose in old plants and hard wood (ligno-cellulose) is of no 

 value as a feeding-stuff. 



16. Qualitative Examination of the Sugars. 



The natural sugars are divided mainly into two groups, the 

 monoses^ possessing the general formula CgHigOg, and the biases 

 of the formula C12H22O11, formed by condensation of two monose 

 molecules, with elimination of one molecule of water. The bioses 

 can be split up into monoses by the addition of a molecule of water 

 (hydrolysis) brought about by the action of enzymes, such as 

 diastase (in malt), ptyalin (in saliva), or by boiling with dilute acids. 



The monoses (glucose, fructose, etc.) occur in largest amount in 

 ripe fruits, while the best-known biose (cane sugar or sucrose), 

 although also present in fruits, exists in larger proportion in the 

 stalks of certain plants such as sugar cane, sorghum cane, and in 

 various roots, especially in those of the sugar beet, the proportion 

 of sugar in certain samples of the latter reaching 25 per cent. 



The following tests should be applied to cane sugar, 

 glucose, and fructose, and also when possible to certain 

 natural sugar-containing substances such as honey, 

 and the aqueous extracts prepared from raisins, sugar 

 beet, and carrots. 



{a) Reaction with HgSO^. — A small quantity of the 

 solid sugar or its syrupy solution in water is heated 

 with concentrated sulphuric acid. Cane sugar is 

 charred, while glucose is not blackened if pure. 



{U) Reaction with Fehlin^s Solution. — Reducing 

 sugars on boiling with Fehling's solution (an alkaline 

 solution of cupric tartrate) ^ give a red precipitate of 

 1 As Fehling's solution deteriorates on keeping, it is advisable 

 to make up the copper solution and the tartrate solution separately, 

 and only mix immediately before using. With the following 

 proportions, the solution can be used for the quantitative estimation 

 of glucose : — Solution A. 17 S^ grams CuS04,5H20 dissolved in 150 

 c.c. water and the cold solution made up to 250 c.c. Solution B. 

 35 grams NaOH and 90 grams sodium potassium tartrate (Rochelle 

 salt) dissolved in 150 c.c. water and the cold solution made up to 

 250 c.c. 



