418 CHEMICAL DISCOVERY AND INVENTION 



glycogen is often referred to as " animal starch," and while it 

 differs from starch in solubility and other properties it appears 

 that under the influence of the same reagents it will, like starch, 

 break up into molecules of glucose. This sugar is excreted in 

 large quantity in the disease known as diabetes. Glucose re- 

 sembles an aldehyde in many reactions and until recently was 

 classed with those compounds. 



Glucoses are widely diffused in plants in the form of com- 

 pounds called " glucosides," many of which are of great practical 

 importance as well as scientific interest. 



The glucosides may be regarded as analogous to fats in con- 

 stitution, being made up of glucose and some complex, alcoholic or 

 acid, the residues of which in the molecule of the glucoside only 

 need the addition of the elements of water to enable them to 

 separate and become independent. Thus a very familiar case is 

 that of amygdaline, a white crystalline substance occurring in the 

 bitter almond. When crushed with water the bitter almond, 

 previously almost odourless, immediately emits a characteristic 

 smell. This is due to the action of an enzyme occurring in the 

 tissues of the almond itself, which in the presence of water 

 causes the decomposition of the amygdaline in the manner 

 expressed as follows : 



Amygdalin 



C 20 H 27 NO n +2H 2 = 



Benzaldehyde Hydrocyanic Acid Glucose 

 C 7 H 6 + HCN + 2C 6 H 12 6 



Among the very numerous natural glucosides the following 

 may be mentioned as possessing great physiological importance 

 in the economy of the plant or yielding products of practical 

 interest and utility. 



Arbutin, a bitter crystalline substance obtained from the leaves 

 of the bear-berry (Arctostaphylos uva-ursi), a small shrubby 

 plant belonging to the heath tribe, is used in medicine. Hydro- 

 lysed by emulsin it yields glucose and hydroquinone which has 

 antiseptic properties. Arbutin occurs in the leaves of various 

 species of pear. 



Phloridzin is a somewhat similar substance found in the root 

 bark of apple, pear, cherry, and plum. 



Salicin is a bitter crystalline substance extracted from the 

 bark of the willow or from poplar buds and is used in medicine. 



