THE VOLATILE PART OF PLAKfS. 51 



Into another test-tube, and add at once to it one-fourth its bulk of 

 iodine solution. The latter portion becomes intensely blue by trans- 

 mitted, or almost black by reflected, light. On standing, observe that 

 In the first case, where starch preponderates, it settles to the bottom, 

 leaving a colorless liquid, which shows the insolubility of starch in 

 cold water ; the starch itself has a purple or red tint. In the case 

 iodine was used in excess, the deposited starch is blue-black. 



By the prolonged action of dry heat, hot water, acids, 

 or alkalies, starch is converted first into amidulin, then 

 into dextrin, and finally into the sugars maltose and dex- 

 trose, as will be presently noticed. 



Similar transformations are accomplished by the action 

 of living yeast, and of the so-called diastase of germinat- 

 ing seeds. 



The saliva of man and plant-eating animals likewise 

 disintegrates the starch-grains and mostly dissolves the 

 starch by converting it into maltose (sugar). It is much 

 more promptly converted into sugar by the liquids of the 

 large intestine. It is thus digested when eaten by ani- 

 mals. Starch is, in fact, one of the most important 

 ingredients of the food of man and domestic animals. 



The starch-grains are not homogeneous. After pro- 

 longed action of saliva, hot water, or of dilute acids on 

 starch-grains, an undissolved residue remains which De- 

 Saussure (1819) regarded as nearly related to cellulose. 

 This residue is not changed by boiling water, but, under 

 prolonged action of dilute acids, it finally dissolves. 

 With iodine, after treatment with strong sulphuric acid, 

 it gives the blue color characteristic of cellulose. There- 

 fore it is commonly termed starch-cellulose. 



Starch-cellulose amounts to 0.5 to 6 per cent of the 

 starch-grains, varying with the kind of starch and the 

 nature and duration of the solvent action. Whether it 

 be originally present or a result of the treatment by 

 acids, etc., is undecided. 



The chemical composition of starch-cellulose is identi- 

 cal with that of the entire starch-grain, viz. : (C 6 H 10 5 )n. 



The starch-grains also contain a small proportion of 

 amidulin, or soluble starch, presently to be noticed. 



