218 PRACTICAL PHYSIOLOGY 



with certain neutral salts, such as ammonium sulphate. They may 

 be sub-divided into three sub-groups, the starches, the dextrines 

 and the celluloses. 



1. The Starches. These include ordinary starch and glycogen 

 (C 6 H 10 5 ) n . Starch is the most widely distributed carbohydrate in 

 the vegetable kingdom, for it is in this form that plants store up 

 their excess of carbohydrate. Animals store their excess of carbo- 

 hydrate partly as glycogen, but mainly as fat. If the amount of 

 dextrose produced in the leaves be in excess of the immediate needs 

 of the plant, it is stored up as starch. 



The exact shape of starch grains varies according to the plant 

 from which they are obtained. In this connection they may be 

 divided into two groups : (1) a group in which the contour of the 

 grains is even, such as wheat, barley, arrowroot, potato ; (2) a 

 group in which the contour is marked by facets, either completely, 

 as in oats and rice, or only partially, as in tapioca and sago. 



EXPERIMENT I. Examine some wheat flour, a scraping of 

 potato, and some ground rice under the microscope. To do this, 

 mix the flour, etc., with a drop of water on a slide, and examine 

 under a cover slip. 



Starch, like most other polysaccharides, is insoluble in cold 

 water, but it swells up in hot water, an opalescent mixture being 

 formed. 



EXPERIMENT II. Place some powdered starch in a test tube, and 

 half fill up with cold water no solution occurs now boil, when an 

 opalescent mixture will be produced, and, if of sufficient con- 

 centration, this will gelatinise on cooling. Try Trommer's test 

 no reduction occurs. 



The standard test for starch is with iodine solution. 



EXPERIMENT III. To an opalescent cold solution of starch add a 

 drop or two of a very dilute solution of iodine in potassium iodide ; 

 a blue colour results, which disappears on gradual heating and 

 returns again on cooling. Excessive heat must be avoided, since 

 the iodine is volatile. 



Starch granules also give this reaction under the microscope. 

 The cut surface of a potato gives it. 



Hydrolysis can be effected by boiling with a weak acid or by the 

 action of ferments such as ptyalin, amylopsin, and malt diastase. 



EXPERIMENT IV. Place some starch solution in a test tube, 

 add to it a few drops of 25 per cent, sulphuric acid and boil for five 

 minutes. Neutralise and apply the iodine test and note that, 

 instead of a blue, a reddish brown colour is produced (due to dextrine). 

 Apply Trommer's or Fehling's test, and note that reduction occurs. 



EXPERIMENT V. Place some of the starch solution in the mouth, 

 and after a few minutes transfer it again to the test tube ; now apply 

 Trommer's or Fehling's test reduction occurs. 



Try the same experiment with some unboiled starch, and note 

 that there is no reduction (i.e. the resistant external layers have not 

 been hydrolysed). 



