EXERCISE 46 



Plant foods are built up in the leaves by means of a kind of manufacturing process. Organic mate- 

 rials are built up chemically out of inorganic compounds. In the process of food manufacture, carbo- 

 hydrates, of which sugar and starch are good examples, are the first substances put together. 

 R Problem. Is the presence of air necessary for starch-making ? 

 What to use. A healthy plant ; some material to keep air from the leaves vaseline, for example. 

 What to do. Keep the well-moistened plant in a dark closet for twenty-four hours ; test a piece of a 

 :af for starch. 1 Coat both sides of a certain portion of a leaf, as a strip down the center, with a thick 

 Kyer of vaseline. Place the plant in sunlight ; at the end of the day, break off the leaf. Test for starch. 

 Record results and state conclusions. 

 NOTE. By careful experiments, scientists have found that it is that part of the air known as carbon dioxid which is 

 ed by plants in starch-making. 



1 To test a leaf for starch, boil it for a few minutes in water, then place for a few minutes in a large test tube containing 

 alcohol (grain or wood) until all the chlorophyl, or leaf-green, has been removed. Immerse in iodine solution. (See Exercise 33 

 for starch test.) Since the fumes of alcohol are inflammable, it is well to carry them away from the source of heat by means of a 

 glass tube in a perforated stopper closing the test tube. 



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