LIVING PARTS OP THE STEM. 75 



of eggs and the sticky part of dough made from wheat flour 

 are good examples. 



Nitric acid turns proteids yellow, and the addition of am- 

 monia afterwards turns them deeper yellow or orange. As 

 few other substances are affected in this way by nitric acid, 

 this change of color is a very good test to show the presence 

 of proteids. 



102. Experiment 21.* Testing an Onion for Proteids. —Test a 

 rather thick slice of onion by heating it in a porcelain evaporating dish 

 with a little strong nitric acid until the latter begins to boil and the 

 onion becomes somewhat softened, i Rinse off the slice of onion in a 

 stream of water, then pour on it a few drops of ammonia water and 

 observe what changes of color (if any) occur. 



Grape sugar is an important substance among those stored 

 for food by the plant. It received its name. from the fact 

 that it was formerly obtained for chemical examination from 

 grapes. Old dry raisins usually show little masses of whitish 

 material scattered over the skin which are nearly pure grape 

 sugar. Commercially it is now manufactured on an enor- 

 mous scale from starch by boiling with diluted sulphuric acid. 

 In the plant it is made from starch by processes as yet 

 imperfectly understood, and another sugar, called maltose, is 

 made from starch in the seed during germination. 



Both grape sugar and maltose (and hardly any other sub- 

 stances) have the power of producing a yellow or orange 

 color and throwing down an orange or reddish deposit, when 

 they are added to a brilliant blae alkaline solution of copper, 

 known as Fehling's solution.^ The color or deposit will not 

 appear until the solution has been heated to boiling. 



1 Do not allow the acid to toucli the hands, the clothing, or any metallic object. 

 If it is desirable to show the result of the test to one or more classes, the portion 



of the onion stained yellow by the acid may be placed in a small wide-mouthed bottle 

 with ground stopper, in which it may be kept for a long time and conveniently 

 passed from hand to hand. 



2 For the preparation of the solution see Appendix B. 



