CHAPTER VII. 



DIFFERENTIATION AND SPECIFICITY OF STARCHES IN RELATION TO GENERA, 



SPECIES, ETC., AS DEMONSTRATED BY THE METHODS EMPLOYED IN THIS 



RESEARCH. 



STARCH-SUBSTANCE A NON-UNIT SUBSTANCE. 



One of the most important problems underlying the differentiation of starches from 

 different sources is whether or not starch is a uniform substance ; and, if not, whether im- 

 portant and diagnostic variations occur under various conditions of plant life; and, if so, 

 the influences of these conditions. Frequent reference has been made to facts which indi- 

 cate, or show, not only that starch is not a unit body, but also that it exists in a large 

 number of forms, and that the starch of any given plant may be regarded as consisting of 

 a mbcture of stereoisomers, which differ not only in their intramolecular structure, but also 

 in their intermolecular arrangements, and hence in their properties. Moreover, it has 

 been shown by the literature quoted that the starch from any given plant is a heterogeneous 

 collection of grains which vary in microscopical and molecular properties, and that the 

 individual grains, except perhaps the embryonic, spherical, and seemingly amorphous 

 grains, are likewise of non-uniform composition. In this research confirmatory evidence 

 of the foregoing statement is abundantly supplied, especially in the reactions, as, for instance : 

 In Pisum, in the starches from Eugenie and Thomas Laxton peas, in which the gi-ains of 

 the rosette type are gelatinized within 5 minutes by chloral hydrate-iodine, while those 

 of the bean type require an hour or more; in J^sculus, Brodicea, Convallaria, Sprekelia, 

 and Wat^onia, in which the small gi'ains react more vigorously with chemical reagents 

 than the large gi-ains; in Fritillaria and Hcemanthus, in which the small grains react less 

 vigorously than the large grains; in Hyacinthus, in which a reversibility of sensitivity is 

 noted in relation to different agents, the small grains \vith regular outline being the most 

 resistant of the thi'ee sizes to chloral hydrate-iodine, the medium-size grains with irregular 

 outline being the most resistive to Purdy's solution, and the small grains the least reactive 

 with iodine; in Lachenalia, in which the small grains are the least reactive with chloral 

 hydrate-iodine, but the most reactive with pyi-ogallic acid, ferric chloride, and Purdy's 

 solution; in Convallaria, in which the small grains are gelatinized at 61 and the large 

 grains at 71.75. Such instances could be considerably multiplied. In fact, every starch 

 affords such illustrations. 



The structure, both histological and molecular, of individual grains of a given starch 

 is not uniform, and this applies not only to compound grains, but also to simple grains, 

 excepting possibly very young grains, before, for instance, the formation of a hilum and 

 the appearance of lamellation. The differences in the behavior of the inner and outer 

 parts, or, according to general ideas, of the so-called amylose and cellulose, can be demon- 

 strated with the greatest ease, and in ways to show that these parts represent different 

 forms of starch-substance. As already repeatedly pointed out, the individualities of these 

 two parts are markedly shown in their different behavior towards various reagents. As a 

 rule, the outer part is the more resistive, but toward some reagents it is the less resistive. 

 In relation to moist heat, when the grains are boiled in water the outer part is always the 

 last to disappear, sometimes resisting boiling for many minutes, appearing in suspension 

 in the form of empty capsules from which the less resistive inner starch has escaped in semi- 

 liquid form and passed into the so-called solution. 



That the capsular or outer part of the normal grain is the last part to be disorganized 

 by heat can readily be proved by observation of grains undergoing gelatinization on the stage 

 of the polarizing microscope between crossed Nicol prisms. Sometimes even a markedly 

 higher temperature is required to gelatinize the outer than the inner part, as in Iris, in which 



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