THE MATERIAL OF PLANTS 19 



18. Tests for the contents of the plant. — If a raw 

 potato is crushed in clear water, the latter becomes 

 milky, and by placing some of it in a small glass 

 and holding up to the window, numbers of minute 

 granules may be seen. They are not more than a 

 hundredth of an inch in diameter. If a few drops 

 of a tincture of iodine obtained from a druggist is 

 added to the water, it turns blue, owing to the 

 action of iodine upon starch. Put a drop of the tinc- 

 ture on the cut surface of the potato and note the 

 result. 



Apply iodine to the cut surfaces of stems and roots, 

 and determine the portions in which starch may be 

 found. 



Sugar in sugar-cane, sorghum, fruits, or the sugar 

 beet, or in the sap of the maple, may be detected by 

 the taste, and the sour acids may be found in the 

 same manner. 



If you were to take the plant to a chemical labora- 

 tory, and make a complete chemical analysis of it, a 

 large number of other substances would be found, 

 such as oils, tannins, alkaloids, proteins, mineral 

 salts, etc. 



Animals, including man, have found uses for many 

 of these substances, and species which produce them 



