SIGNIFICANCE OF GLUCOSIDES 173 



exception of sugar, are frequently of a volatile nature and 

 possess a characteristic smell ; their chemical nature, how- 

 ever, varies so much that there is no general test for the whole 

 group other than that afforded by the presence of a carbo- 

 hydrate after hydrolysis. 



The method devised by Bourquelot has been much em- 

 ployed ; the expressed sap of the plant is examined as to its 

 power of reducing Fehling's solution and as to its optical pro- 

 perties, the ferment emulsin is then added to the extract and 

 the mixture kept in the warm for a time. The amount of 

 sugar is thereupon again estimated and so also is its rotatory 

 power. The increase indicates roughly the amount of glu- 

 coside originally present. 



In microchemical work the same test, in a simplified form, 

 may be applied, the preparation being treated with a solution 

 of emulsin or of dilute sulphuric acid, and then gently warmed 

 with Fehling's solution. 



These tests, however, are of value only in certain cases, 

 for some glucosides, and for that matter some other sub- 

 stances likewise, can reduce Fehling's solution without under- 

 going a preliminary hydrolysis, also there may be present in 

 the cells of the plant other substances which on hydrolysis 

 would yield glucose, and further some glucosides like gyno- 

 cardin exhibit a remarkable resistance to the action of acids. 



The decomposition products of many glucosides are 

 brightly coloured, for example, rhinanthin when boiled with 

 dilute hydrochloric acid yields a dark blue-green colour which 

 is due to rhinanthogenin. Several give a brilliant red color- 

 ation with strong sulphuric acid, e.g. salicin and phloridzin. 

 Most glucosides are soluble in varying degrees in either hot or 

 cold water and alcohol ; the majority are insoluble, or nearly 

 so, in ether, which fact is made use of in separating them 

 from alcoholic solutions. 



PHYSIOLOGICAL SIGNIFICANCE OF GLUCOSIDES. 



In attempting to assign the part played by these sub- 

 stances in the economy of the plant, it must be remembered 

 that the number of glucosides of natural occurrence are very 

 numerous and, in some cases, of a diverse nature ; it is, there- 

 fore, possible that the significance of the presence of one glu- 



