THE VOLATILE PART OF PLANTS. 65 
Exp. 26.—Shake together ina test tube, 30 c. c. of water and starch 
of the bulk ofa 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 1 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 30c¢. ¢. 
of cold water and heat to boiling. ‘The starch is converted into thin, 
translucent paste. That a portion is dissolved, is shown by. filtering 
throuzh 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. 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 asmall proportion of matter not readily soluble in this 
liquid. This remains as 4 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. ; 
