876 



STARCHES OF ETJPHORBIACE^. 



GENUS MANIHOT. 



This genus includes about 80 species of American perennial herbs and shrubs which are dis- 

 tributed chiefly in BrazU and Mexico — Manihot, Manioc, and Manioca, as the plants are variously 

 called, and the cassava plant. M. utilissima Pohl, or the bitter cassava, is cultivated extensively, 

 especially in Brazil, where starch is obtained from the tuberous roots, which sometimes weigh from 

 15 to 20 kilograms, and is prepared by drying on heated plates. This starch is sold as tapioca, also 

 known as BraziUan arrowroot. (See Marardaceoe, page 813.) During this heating many grains are 

 partially or completely gelatinized. 



STARCH OF MANIHOT UTILISSIMA (COMMERCIAL TAPIOCA). (Plate 98, figs. 585 and 586. Chart 386.) 



Owing to the injury to most of the grains by heating during the preparation of the starch, 

 the grains are not normal in form or in their behavior with reagents. 



Histological Characteristics. — In form the normal grains, which may be separated from the 

 partially gelatinized masses, are simple and rarely found in aggregates of two or three components. 

 They are fairly regular in outline and with no prominences or spicules. All of the isolated grains 

 show pressure-facets, usually one or two or rarely three or four. The conspicuous forms are the dome- 

 shaped to hemispherical; also spherical, ovoid forms with one or two facets, and polygonal forms 

 with three or more facets. There are gelatinized and partly gelatinized grains, which may be readily 

 distinguished from the non-gelatinized by their clearness, great size, and irregularity of shape, 

 especially at the faceted ends, where there is especial 

 prominence of the comers. 



The hilum varies greatly in size. In the fully gelatin- 

 ized grains it is not visible. In those partly gelatinized it 

 varies in size according to the degree of gelatinization. 

 In those apparently unaffected it is comparatively large 

 and distinct, centrally placed, and slightly fissured, often 

 in a stellate fashion. Often the ungelatinized grains show 

 a concavity at the facets which appears to extend as far 

 as the hilum. 



The lamellae are indistinct, roimd, rather coarse, reg- 

 ular, concentric rings, one or two of which may be larger 

 than the others without much regard to position. There 

 are about 8 to 10 lamellae on a medium-sized grain. 



The ungelatinized grains vary in size from 5 to 20>i. 

 The common size is about 15^. 



Polariscopic Properties. — In the ungelatinized grains 

 the figure appears centric or slightly eccentric and dis- 

 tinct. The lines are of very much the same size and 

 distinctness throughout their length. 



The degree of polarization is fairly high, varying according to the aspect of the grain viewed. 

 If the hilum is partially gelatinized the grain is not polariscopic in the center, and the completely 

 gelatinized grains have entirely lost their polarization. 



With selenite the unaffected grains vary in the definition of the quadrants, which sometimes are 

 very well defined and at other times not well. They may or may not be regular in size or shape. In 

 the unaffected grains the colors are fairly pure. 



Iodine Reactions. — The gelatinized and partially gelatinized grains were colored an indigo in 

 varying degrees. The apparently unaffected grains showed very little blue-violet coloration even 

 with a 0.5 per cent solution. After heating in water until all the grains were completely gelatinized, 

 the solution was colored deeply and the grains faintly with iodine; some of the grains were reduced 

 to granular masses. After boiling for 2 minutes, the grain-residues do not color at all except on 

 adding excess of iodine, when the capsules becomes a pinkish-violet. Most are more or less reduced 

 to granular masses. 



Staining Reactions. — With gentian violet there is a slight reaction in 3 minutes, but after 25 

 minutes the normal grains are stained only faintly. 



