Vcf/etable Physiology. 399 



the outer cells of the seed are filled with starch-granules so densely 

 packed that they are flattened ag-ainst one another (Z>), and present 

 angular forms, fitted together like stones in a wall ; they all pre- 

 sent rather a large central cavity. In rice (fig. 15), where the 

 granules are exceedingly small, they are packed in the same way, 

 but even more densely, whence arise the horny character of this 

 seed, and the granular nature of its flour, in which the starch- 

 granules are but imperfectly separated from each other. 



The starch occurring more sparingly in the cells of vegetative 

 tissues appears both in the simple and the compound form, the 

 granules being usually far smaller than in the above-described 

 examples — often so small that their nature can only be determined 

 after causing them to swell up by applying certain reagents. The 

 microscopist's universal test for starch, where the size or appear- 

 ance of granules leaves their nature in doubt, is iodine, which 

 colours the starch-granules a more or less deep violet or blue, 

 according to the strength of the solution. Alcoholic tincture of 

 iodine is often used ; but this is apt to deposit crystals of iodine 

 in the water in which the object lies, so that a solution of iodine 

 in an aqueous solution of iodide of potassium is more convenient. 

 When starch-granules have been previously treated with dilute 

 sulphuric acid (which is done to swell them when very minute), 

 the blue colour inclines to purple, more or less reddish, according' 

 to the amount of action of the acid. The nature of the combi- 

 nation of iodine and starch is yet unexplained by chemists, but 

 the fact of the colouring produced is of the highest interest, not 

 only on the ground of its value as a test, but from the relations 

 indicated to the softer forms of cellulose above described. Starch- 

 granules extracted from the potato, for instance, and viewed free 

 in water, appear as solid bodies insoluble in cold water. They 

 are found to contain a variable proportion of water in their 

 substance according to the extent to which they are dried. The 

 statement that they are not altered by cold water is only abso- 

 lutely true of the external denser lamellae, since we have found 

 that, by crushing freshly-extracted granules, and allowing water 

 to have direct access to the inner lamellae, these occasionally 

 swell out to some extent by absorption of water, and deform 

 the grain, the outer lamellae resisting the action. When the 

 water is heated (and applying diluted sulphuric acid or solu- 

 tion of potash has the same effect), the granules swell, the streaks 

 disappear, the internal substance softens into a jelly, and the 

 whole swells up into a large gelatinous mass, having, how- 

 ever, still a definite outline. The phenomena here exhibited 

 vary a little. Sometimes the softening and swelling are uniform ; 

 the whole granule is then as it were blown out into a gelatinous 

 bubble, probably from endosmose of water, and the granule is thus 



