THE VOLATILE PART OF PLANTS. 65 



EXP. 26. Shake together in a test tube, 30 c. c. of water and starch 

 of the bulk of a kernel of maize. Add solution of iodine, drop by drop, 

 ' agitating until a faint purplish color appears. Pour off half the liquid 

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

 solution. The latter portion becomes intensely blue by transmitted, 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. 



EXP. 27. Place a bit of starch as large as a grain of wheat in 30 c. c. 

 of cold water and heat to boiling. The starch is converted into thin, 

 translucent paste. That a portion is dissolved is shown by filtering 

 through paper and adding to one-half of the filtrate a few drops of iodine 

 solution, when a perfectly clear blue liquid is obtained. The delicacy 

 of the reaction is shown by adding to 30 c. c. of water a little solution 

 of iodine, and noting that a few drops of the solution of starch suffice to 

 make the large mass of liquid perceptibly blue. 



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

 or alkalies, starch is converted first into dextrin, and finally 

 into sugar (glucose), as will be presently noticed. 



The same transformations are accomplished by the action 

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

 ing seeds ; see p. 328. 



The saliva of man and plant-eating animals usually 

 likewise dissolves starch at blood heat by converting it in- 

 to sugar. It is much more promptly converted into sugar 

 by the liquids of the large intestine. It is thus digested 

 when eaten by animals. It is, in fact, one of the most im- 

 portant ingredients of the food of man and domestic ani- 

 mals. 



The action of saliva demonstrates that starch-grains are not homoge- 

 neous, but contain a small proportion of matter not readily soluble in this 

 liquid. This remains as a delicate skeleton after the grains are other- 

 wise dissolved. It is probably cellulose. 



The chemical composition of starch is identical with 

 that of cellulose ; see p. 60. 



Air-dry starch always contains a considerable amount 

 of hygroscopic water, which usually ranges from 12 to 20 

 per cent. 



