i6o THE CARBOHYDRATES 



2. The most characteristic reaction of starch is the blue 

 colour produced with iodine. The composition of this blue 

 substance varies ; it contains, on an average, about i8 per cent 

 iodine, and cannot be formed unless a small quantity of hydri- 

 odic acid, which is always present in small amounts in ordinary 

 solutions of iodine, be present. The blue colour is discharged 

 on heating the solution, but reappears on cooling. The dried 

 substance may, however, be heated to ioo° without under- 

 going alteration. 



If the starch grains are very small, or relatively so few in 

 number that they might easily be overlooked, Meyer's pro- 

 cedure for their detection may be followed. A section of the 

 material to be examined is cut, and is first treated with a 

 fairly dilute solution of iodine in potassium iodide, the excess 

 of the reagent is then removed, and the section is irrigated 

 with a concentrated aqueous solution of chloral hydrate. This 

 causes the starch grains to swell, and at the same time the 

 other cell contents are dissolved, as are also the starch grains 

 in time. 



The fact that iodine sometimes gives a blue colour with 

 a soluble cell constituent led to the assumption of the 

 presence of a so-called soluble starch. There is, however, no 

 need for such an interpretation, since the blue colour ob- 

 served in the epidermal cells of Saponaria officinalis, for 

 example, is attributable to the action of iodine on the glucoside 

 saponarin, CaiHaiOia, which Barger * has shown to be present 

 and to give this reaction. 



The blue colour given by starch with iodine was originally 

 regarded by Mylius f as a definite chemical compound, and 

 the same view is taken by Murray $ but by others it is con- 

 sidered to be a physical adsorption of colloidally dispersed 

 iodine by the starch acting as a protective colloid ; § the 

 particular shade of colour produced probably depends upon 

 the degree of dispersion of the iodine (cf. dextrin, glycogen, 

 etc.). 



•Barger: " Ber. deut. chem. Gesells.," 1902, 35, 1296. 



t Mylius : id.. 1887, 20, 688. 



I Murray : " J. Chem. Soc," 1925, 127, 128S. 



§ Barger and Field : id., 1912, loi, 1394- 



