24 



METHODS USED IN THE STUDY OF STARCHES. 



2-inch eye-piece, or a ^-ineh objective and a 2-ineh eye- 

 piece, giving magnifications on the field of projection of 

 72, 180, and 300 diameters, respectively. 



5. Keactions in Polaeized Light Without and 

 With Selenite. 



■Starches have been found to exhibit not only marked 

 differences in the degrees with which they rotate the 

 plane of polarized light, but also differences in the 

 characteristics of the " interference figure " or " cross," 

 as it is generally termed. The general characteristics, 

 distinctness, shape, regularity, and position of the inter- 

 ference figure, and also the approximate degree of aniso- 

 tropy or intensity of polarization were readily studied. 

 By the aid of selenite it was determined whether the 

 optic properties were negative or positive, and also the 

 size, shape, and regularity of the quadrants, as well as 

 the intensity and pureness of the blue and yellow colors. 

 In spherical grains with centrally located hila, the two 

 parts of the " cross " intersect at the hilum, or mathe- 

 matic center, of the grain, so that the term quadrant 

 has a proper application ; but in the case of grains having 

 eccentric hila the position of the point of intersection of 

 the two parts of the cross, together with their curvatures, 

 may destroy every semblance of quadrants according to 

 the conventional definition of this word. This term has 

 therefore been used in a very broad sense throughout 

 our investigation to indicate the four parts of the grain 

 that are defined by the two parts of the cross, in prefer- 

 ence to the great multiplicity of terms that would be 

 required to define these parts if great accuracy were 

 attempted. Likewise, for convenience we have referred 

 to the "lines" of the interference figure in preference 

 to the " arms " of the cross. 



All starches are " optically negative," hence no special 

 references have been made in the text in this particular. 



The slides for polariscopic examination are prepared 

 as follows : The end of a small spatula is thrust into the 

 specimen of starch and moved about, withdrawn and 

 sharply tapped several times in the center of the slide, 

 and the slide jarred in a manner to cause a practically 

 imiform distribution of the starch grains in a single 

 well-disseminated layer. The margins of this layer are 

 carefully removed so as to leave an area 12 mm. square. 

 An expeditious way of removing the margin so as to in- 

 sure a uniform area of starch is to use as a wiper a piece 

 of sheet celluloid having a 12-mm. slot, wiping trans- 

 versely and then longitudinally. A couple of drops of 

 balsam are carefully added at the center of the area, 

 a cover-slip put on, and the slide placed on the stage of 

 the polarizing microscope. After determining the degree 

 of polarization, the selenite plate is introduced and the 

 specimen again examined. 



In order to reduce the degree of polarization into 

 values in comparative terms and figures it was found 

 desirable to adopt an arbitrary scale (Chart B 2, Chapter 

 IV), and to select three starches as standards that give 

 wide and properly separated gradations of value. Thus, 



adopting a scale of 100 divided primarily into units of 5, 

 the starch of Solanum tuberosum was taken as having a 

 value of 90 and " very high" ; that of Na/rcissus poeticus 

 ornatus as having a value of 50, or " moderate " ; and 

 that of Richardia albo-maculata as having a value of 30, 

 or " low." Intermediate gradations are readily expressed 

 by both words and figures. If the starch examined has, 

 for instance, the same degree of polarization as that of 

 Narcissus poeticus ornatus it is given a value of moderate 

 or 50, but if its value be between moderate (50) and 

 high (70) it is recorded as being moderately high (60), 

 or moderate to moderately high (55), or moderately 

 high to high (65). In some instances intermediate 

 values are given where it is necessary to express smaller 

 differences, as between members of a set consisting of 

 parents and hybrid. The different grains of any given 

 specimen of starch vary in the degree of polarization, 

 so that in rating the average must be estimated; as a 

 consequence all of the records are averages. The method 

 is of a very gross character and the personal equation 

 in determining values may be very important and lead 

 to more or less divergent records by different observers, 

 but in practice it has been found that after a degree 

 of skill has been acquired, as is common in all such 

 gross methods of experiment, essentially or absolutely 

 the same values are recorded when experiments are re- 

 peated several times at well-separated intervals, or made 

 by two individuals who have had practically the same 

 training. Owing to variations in illumination from time 

 to time, it is quite important to use persistently, in con- 

 junction with the starch to be examined, some starch 

 that has been adopted as the standard of comparison, 

 preferably one that has a close value. Thus, when 

 studying the starches of a group, one of the starches is 

 standardized with the starch-standard and scale adopted, 

 as before stated, the standard recorded for this starch 

 serving as the fundamental standard for comparison for 

 the others of the group. This method gives very good 

 comparative results, especially when the group consists 

 of a few members; but it is, on the whole, the least 

 valuable of all the methods employed in this research, 

 and its usefulness is chiefly because of its remoteness 

 from the characters of the other methods. 



6. Iodine Reactions. 



The use of iodine not only served to bring out certain 

 histological peculiarities, but also valuable data in the 

 differentiation of different kinds of starch. The typical 

 or ordinarily observed reaction of starch with iodine is 

 an indigo-blue, but if an excess of iodine be avoided 

 the reaction of the grains will be found to vary usually 

 from a blue to reddish-violet, including within these ex- 

 tremes all shades of violet from a purple to a reddish- 

 violet according to the kind of starch. In fact, in the 

 presence of minute quantities of iodine, starches are 

 colored some shade of violet, varying with the kind of 

 starch. With any quantity of iodine certain starch- 

 grains yield a red reaction. In studying the iodine reac- 



