The Various Substances Made by Plants 109 



composed of it. Occurring as a rule in tiny white grains scattered 

 widely through all kinds of tissues, it collects in some organs, 

 which swell very greatly for its reception. Such is the Potato, 

 which is simply a starch-storing underground stem: the Sweet 

 Potato, a starch-storing root: bulbs, which are masses of starch- 

 filled leaves: and most seeds, including all of the grains, which 

 contain copious starch either inside or around the embryo. In 

 all of these cases, starch presents a characteristic homogeneous 

 firm whitish appearance, contrasting markedly with the soft 

 translucent aspect of structures in which the food is stored up as 

 sugar, e. g., the Sugar Beet, Sugar Corn. It happens, however, 

 that its presence can be detected in a very conclusive way, namely 

 by the deep blue color it assumes when touched by a solution of 

 iodine, as the reader already has learned, and as he can easily 

 prove for himself by applying a little of the tincture of iodine to a 

 lump of starch from the laundry box, or to a disused cuff, or to 

 water in which some starch has been scraped, — and heated until 

 it forms a fine paste. The test is one of the most satisfactory 

 and important in all organic chemistry, and so delicate that, by 

 its use with the aid of the microscope, one can detect even the 

 minutest quantities of starch in the tissues of a plant, where it is 

 sometimes distributed with a curious and beautiful geometrical 

 exactness. It is necessary to warn the experimenter, that in 

 living tissues, however, the test often works rather badly, because 

 iodine penetrates active protoplasm very slowly. 



Starch, when it accumulates in the plant, serves as a store of 

 reserve food upon which the plant can draw when it starts new 

 growth; and starch is by far the most connnon and abundant of 

 plant foods. Moreover, it serves equally well as a food for an- 

 imals, which, accordingly, rob the plants; and these are there- 

 fore obliged as a whole to make a huge surplus in order to keep 

 any at all for themselves. The importance of starch as food for 

 man is evident when one recalls that Wheat, Corn, Rice, Barley, 

 Rye, — grains, which constitute the principal food of the great 



