VALUES OF METHODS IN THE DIFFERENTIATION OF STARCHES. 307 



form, as is evident in the fact that even a true solution of starch is active although to 

 a relatively very minor degree. 



It is questionable if much value is generally to be attached to variations in the degree 

 of polarization of different starches in the recognition of differentiations of different forms 

 of star('li, because such differences are, as a rule, very closely related to the thickness of 

 the grains and variations in the form; and in the absence of exact measurements may be 

 entirely misleading. By means of the polarizing microscope the "interference figure" or 

 "cross," and the color-effects of starch in the presence of the selenite plate, may be studied, 

 and also with the latter certain structural peculiarities; but these means are useful only 

 with reference to the grosser characteristics of starches, rather than with the recognition of 

 different kinds of starch-substances, and on the whole, while having a certain individuality 

 and distinct usefulness, seem to have a distinctly less value than other available methods. 



THE IODINE METHOD. 



Since the discovery of the starch-iodine reaction by Stromeyer this test has been the 

 recognized standard without interniption to the present time; but certain popular mis- 

 conceptions have become attached to the reaction, as for instance, that starch, whether 

 in the form of grains or in solution, always gives a blue reaction with iodine, and that the 

 blue reaction is diagnostic of starch. There are forms of cellulose, glycogen, and plant 

 mucus that give a blue reaction with iodine, and normal starch-grains may give color 

 reactions ^'arying from an intense indigo-blue to purple, bluish-violet, violet, red-violet, 

 old-rose, heliotrope, wine-color, etc. Similar differences in reaction may under appropriate 

 conditions be oliserved in the starch-grains //( situ. 



Differences of this kind in relation to different starches have been beautifully demon- 

 strated by Dubosc (page 171) by means of iodine ^'apor, corn starch becoming colored a 

 blackish-\iolet, wheat starch a bluish-gray, sago starch a brownish-gray, and potato starch a 

 yellowish-gray, the amount of yellow in the latter being proportional to the amount of foreign 

 matter. With weak solutions of iodine, such as were used in this research, the grains almost 

 invariably color a bluish-violet, violet, or reddish-violet; and when grains are subjected to 

 moist heat to bring about complete gelatinization and the rupture of the capsules without 

 their disintegration, the starch which has passed through the capsules and passed into solu- 

 tion gives with rare exceptions, as noted under red-reacting starches, a l)lue reaction, and 

 likewise any starch that may be retained within the capsules; but the capsules themselves 

 always, with the exceptions noted, are colored some tone of a blue-red, or a red or brown, etc., 

 as stated on page 303. Hence we may speak of the intracapsular starch as a blue-reacting 

 starch, and of that constituting the capsules as a violet-, or red-, etc., reacting starch. 



The starch-grains from some sources become red on the addition of iodine, which has 

 led to the assumption that the grains consist of erythrodextrin instead of starch; and in 

 the case of grains which yield a violet reaction, it has been taken for gi'anted that the red 

 element of the blue-red color is likewise due to erythrodextrin, or that a specific individual 

 substance which we may term amylodextrin is the reacting substance. As heretofore stated, 

 there are not sufficient reasons for the assumption of the existence of the latter, since all 

 of the so-called amylodextrin reactions can be accounted for upon the basis of a modified 

 form of starch or of contamination of starch with erytlu'odextrin. 



The nature of the substance of starch-grains which give a red (not a violet) reaction with 

 iodine is yet problematical. Apart from the color reaction, such grains behave like ordinary 

 starches when subjected to the other processes used in this research. Thus the grains of A lo- 

 casia putzeysi color a wine-red in the presence of iodine, and if they are boiled in water until 

 the capsules rupture, the solution and the capsules color red, but otherwise yield typical starch 

 reactions. Starches which give a reddish-violet reaction, such as those of Oryza saliva, must 

 not be confounded with grains which, like those of Alocasia putzeysi, give a red reaction. 



