CHAPTER XXXII 



PLANT PRODUCTS 



THE purpose of this lesson on plant products is to 

 lead the pupils to discover the various substances found in 

 plants and to prepare them for the lessons in soil chemistry 

 which follow. 



Have the pupils scrape as fine as possible one or two 

 potatoes. Place the scrapings in a tumbler of water, stir 

 thoroughly two or three times, and set aside to settle. 

 Examine the next day. What do you find in the bottom 

 of the tumbler ? Drain off all the water and potato pulp, 

 leaving nothing but the starchy-looking mass in the bottom. 

 Boil some water over the alcohol lamp and pour a little of 

 this into the tumbler, stirring until the starch thickens. 

 This resembles ordinary starch used for clothes. There 

 is a chemical test, however, that will prove beyond any 

 doubt that this is starch. Place a small quantity of the 

 starch on a plate or saucer, and then put two or three drops 

 of iodine on it. Tincture of iodine that may be bought 

 at any drug store will serve the purpose. This may be 

 diluted with water. What is the effect of the iodine on this 

 substance? The blue color indicates the presence of the 

 starch. The darker the blue the more starch is present. 

 Sometimes it is almost black. Place a drop of iodine on a 

 slice of potato. Does it show as much starch as that 

 which was cooked? The reason the latter shows more 



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