THE WORK OF LEAVES 169 



extract (called malt) is made; yeast is then added, by 

 the action of which the sugar is changed to alcohol. 



It is generally found that seeds which contain starch 

 contain also diastase, and that during germination the 

 starch is more or less completely changed to sugar and 

 transported to the places where it is needed. The cells 

 through which it wanders usually contain starch, owing 

 to the fact that the sugar is frequently changed back 

 to starch again, probably as often as it reaches a cer- 

 tain concentration in the cell. For this reason it is 

 comparatively easy to trace the -path from the seed- 

 leaves to the growing parts of the plant by cutting the 

 plant into halves lengthwise and applying iodine solu- 

 tion to the cut surfaces. Trace the path of the starch 

 in this way in the seedlings you have at your disposal. 



The power of changing starch into sugar, which, 

 in human digestion, is begun in the mouth, is com- 

 pleted in the small intestine, into which the food 

 passes after leaving the stomach. The pancreas (or 

 sweetbread) pours into the small intestine the pan- 

 creatic juice, containing different sorts of ferments, 

 which act, one on starch, another on fats and others 

 on the proteids. 



In order to study the behavior of fats, take a little 

 fat or oil in liquid form (melted butter, cotton-seed 

 oil, olive oil or cocoanut oil) and mix it with water; 

 shake it up in a bottle until the mixture becomes milky 

 in appearance. Allow it to stand and notice the sepa- 



