IS6 CHEMICAL COMPOSITION OF PLANTS 



Sugar-cane stems contain from 15 to 20 per cent., the sugar- 

 beet from 1 2 to 1 6 per cent, of this carbohydrate. 



It differs from the two previous sugars in that it does not 

 reduce Fehling's solution and cannot be fermented directly by 

 yeast. When boiled with dilute acids or acted upon by the 

 enzyme invertase, which is present in yeast and in various 

 tissues of plants, it decomposes into a mixture of dextrose and 

 levulose which mixture is termed invert-sugar. 



Ex. 81. — Boil some pieces of mangel or sugar-beet in water and 

 (i) Test some of the solution for a ' reducing ' sugar as in Ex. 80. 

 (ii) Take 10 c.c. of the solution and add to it three or four drops of strong 

 hydrochloric acid : boil for twenty minutes, and after neutralising the acid 

 with a solution of sodium carbonate, boil and test again with Fehling's 

 solutions. 



(iv) Maltose (C12H22O1J) is a variety of sugar formed by the 

 action of the enzyme diastase upon starch and is present in 

 malted barley and other germinated grain. It is capable of 

 direct fermentation by yeast, and reduces Fehling's solution but 

 not to the same extent as grape-sugar. 



b. Starch (CgHi(|05)„.— -This carbohydrate is found in the 

 form of minute solid, organised grains, built up of several 

 successive layers of the substance arranged round a more or 

 less central nucleus or hilum; sometimes two or more nuclei 

 are visible in the same grain in which case the latter is described 

 as compound. 



Starch-grains are usually manufactured by the plastids of the 

 cells, and occur in greatest abundance in roots, tubers and seeds 

 where they form a store of reserve-food : from 50 to 70 per 

 cent, of the dry weight of cereal grains, and 10 to 30 per cent, 

 of potatoes is starch. 



The grains are variable in size and form even in the same 

 plant : nevertheless, in many cases the starch-grains from certain 

 plants are so characteristic in shape and dimensions that they 

 may be readily identified under the microscope. 



