3UG DIFFERENTIATION AND SPECIFICITY OF STARCHES. 



because, during crystalline growth, of pressure or other conditions which may give rise to 

 morphological dei)artures in various directions. 



If, however, it should be found that definite relationships exist between the histo- 

 logical, polariscopical, and chemical reactions, the histological data would prove of indirect 

 and cooperative value in determining the particular form or forms of starch-substances 

 present, and in indicating inherent peculiarities of the protoplasmic processes concerned 

 in starch-formation. Throughout this investigation only sufficient attention has been 

 given to the gross histological characters of the different forms of starch to permit, if 

 possible, the recognition of each starch; to jiermit of gross comparisons; and to show the 

 likenesses of the starches of the members of a genus, and of closely related genera, and 

 of the likenesses and unlikenesses of the ^'arious genera assigned to a given family, etc. 

 As the research progressed it was found that closer examination often brought out features 

 of importance, and it is to be regretted that throughout the work the microscopical studies 

 were not carried out more in detail, especiallj' with the aid of aniline dyes and other micro- 

 scopical reagents. (See Prefatory Notes in Part II.) 



THE POLARIZATION METHOD. 



The behavior of starch-grains in polarized light was discovered by Biot in 1844, or 

 likely some years earlier, since he and Persoz stucUed the polariscopical properties of dex- 

 trin in 1833. Starch-grains in whole or in fragments are tloubly refractive or anisotropic, 

 and hence rotate or polarize the plane of polarized light from one plane to another. The 

 extent of this rotation is indicated, other things being cciual, by the degree of illumination 

 of the starch-grain when viewed between crossed Nicol prisms. The brightness, or 

 "degree of polarization," or intensity of reaction, varies with the coefficient of the aniso- 

 tropic substance and directly as the thickness of the layer of substance through which the 

 light passes. Hence, when the grains are viewed on edge or end, usually the thicker diame- 

 ter, they accordingly appear to have a higher degree of polarization than when viewed 

 on the flat or through the narrower diameter. When grains overlap, the overlapped 

 parts are brighter than the other parts; and if grains vary in thickness in any aspect, the 

 thicker the part the higher usually the illumination. In all starches the grains ^'ary in 

 size; hence, other things being equal, the larger the grain (or, in other words, the thicker 

 the grain) the higher the polarization. Hence in rating the relative degrees of polariza- 

 tion of different starches, the average effect is taken as the index for that starch. 



The property' of anisotropy or optical activity depends upon internal molecular structure 

 or upon intermolecular arrangement, or both. In the case of substances which are aniso- 

 tropic in solution as well as in solid form, the property is primarily inherent in the molecules 

 and due to peculiarities of the internal structure of the molecules, to which may be added 

 secondarily anisotropy due to external molecular arrangement. This is the case with such sub- 

 stances as sugar, which, whether in solution or in crj^stalline form, rotate the ray of polarized 

 light. But if the substance is anisotropic only when in crystalline form as in sodium chlorate, 

 the property is dependent upon the arrangements of the molecules in relation to one another; 

 hence, if the structural arrangement be destroyed by solution, gelatinization, etc., they lose 

 their property of anisotropy and are said to be optically inactive. 



If the starch-grains, or fragments of grains, be placed on the stage of the polarizing 

 microscope between crossed Nicol prisms, and gradually heated in water, or subjected 

 to the action of a solvent or gelatinizing chemical reagent, it will be observed that the 

 moment gelatinization begins at any part of the grain, the part of the grain that is affected 

 loses, apparently entirely, its property of polarization because the intermolecular arrange- 

 ment is destroyed. This seeming complete loss of optical activity has been utilized in this 

 research in the determinations of the "temperature of gelatinization." But the optical 

 activity of starch is not due solely to intermolecular arrangement of the particles in solid 



