104 TEXTBOOK OF BOTANY 



Protein granules may be distinguished from fat droplets and starch 

 grains by a number of different stains; but perhaps a better concept of 

 proteins may be obtained by kneading a small amount of wheat flour 

 in water until all the starch grains have been washed from the dough. 

 Sugars also are present in plant cells, but they are usually in solution in 

 the cell and are invisible. Their presence may be detected by tasting 

 the plant, or by placing a section of the plant organ in a drop of 20 per 

 cent sodium hydroxide in which a few crystals of copper tartrate have 

 dissolved. If a reddish-yellow precipitate of cuprous oxide is obtained, 

 the presence of sugar is indicated. The reaction may be hastened by 

 heat (40° C). To detect cane sugar by this method the plant section 

 should first be treated for several minutes with the enzyme invertase or 

 with a dilute acid, for instance 5 per cent citric acid. 



The cell walls are made principally of cellulose and pectic compounds, 

 though some of them may also contain a substance (lignin) character- 

 istic of wood, or a fat-like substance ( suberin ) characteristic of cork. 



All the substances mentioned above have been extracted from plant 

 cells bv chemical methods, and most of them may be obtained on the 

 market in a relatively pure state. Thus one may readily buy several kinds 

 of sugar ( especially sucrose, glucose, and fructose ) , starch, several kinds 

 of plant oils, pectic compounds (Certo), and cellulose (filter paper); 

 and then apply to them the tests that were proposed above for detecting 

 the same substances in plant cells. Some of these substances we readily 

 recognize as constituents of our daily diet. 



Sugars. Each of the sugars mentioned above mav be sold under any 

 one of three names. Glucose (CoHi^Oo), also known as grape sugar and 

 dextrose, is not very sweet, but it is one of the principal sugars in plants 

 and also in the blood stream of animals — in fact, it is sometimes called 

 "blood sugar." Fructose (CuHi^Ot;), also called fruit sugar and levulose, 

 is one of the sweetest of sugars and is common in plants.^ Sucrose 



^ The apparent anomaly of two sugars ha\'ing identical formulas (CaH,.Oe), but differ- 

 ing in chemical properties, may be explained by the differences in the arrangement of the 

 atoms. Following are diagrams showing these differences in glucose and fructose molecules. 



H H 



H H H () H H H H H O H 



I I I I I I I I I I 



H— C— C — ( '— ( '— ( — C=<) H— C— C— ("— C— C— C— H 



O OHO O () O H O O 



H II II II H II II H 



Glucose Fructose 



These diagrams of sugar molecules are introduced here merely to help visualize differences 

 in the arrangement of atoms in the simple sugars, not to memorize. 



