348 MICROCHEMISTRY OF PLANT PRODUCTS 



tate of cuprous oxide will be thrown down. This reaction is 

 characteristic of dextrose, maltose, lactose, laevulose and many 

 glucosides. In this instance, however, we are dealing with dex- 

 trose. This reaction may also be carried out on the microscope 

 slide. Sections from the pear three or four cell-layers thick 

 should be placed on the slide in a few drops of the solution, the 

 coverglass should then be put on, and the solution heated until 

 bubbles begin to arise. The microscope will then reveal the gran- 

 ular precipitate of cuprous oxide within the cells. Portions of 

 the leaf of the Balsamina may be treated on the slide as directed 

 for the sections from the pear. See under Fehling's Solution 

 and Copper Acetate in the last chapter. 



Glucosides. — See page i8o, and under Amygdalin, Betulin, 

 Datiscin, Frangulin, Hesperidin, Indican, Potassium Myronate 

 (under Myrosin), Phloridzin, Ruberythric acid, Rutin, Salicin, 

 Saponin, Solanin, Syringin. 



Glycogen, CgHjoOg. — This is a colorless, amorphous, highly 

 refractive substance occurring quite commonly in the cells of 

 fungi. It is soluble in water, but within the cells it may be 

 stained a reddish-brown by means of iodine. 



Gums. — These are amorphous, transparent substances which 

 dissolve in water more or less completely and form a sticky solu- 

 tion. They may be precipitated from their aqueous solutions 

 by alcohol. Those gums which dissolve in water completely, 

 such as the gum of the cherry, apricot, peach, and gum arabic, are 

 called true gums, while those which contain cellulose and are not 

 completely soluble in water are known as mixed gums. Gum 

 tragacanth is an example. One of the most striking characteris- 

 tics of gums which may be used in their identification is their 

 great capacity to swell in water. To follow the process of swel- 

 ling with the microscope, sections should be cut from dry material 

 with a razor which may be wetted with alcohol, but not with water. 

 The sections should be placed on a slide in a drop of strong 

 alcohol, the coverglass should be put on, and a drop of water 

 placed on the slide so that it just touches the edge of the cover- 

 glass. As the water mixes with the alcohol and comes in con- 



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