48 GLUCOSIDES 



of so-called reserve-celluloses, polysaccharides which differ some- 

 what from ordinary cellulose and break down more readily into 

 simple sugars. 



Of very common occurrence in plants are complex compounds 

 known as glucosides, which, in the majority of cases, consist of 

 glucose combined with one or more aromatic substances or other 

 organic residue. On boiling with dilute mineral acids they split 

 up into their constituents (hydrolysis). The decomposition of 

 the glucoside within the plant is effected by special ferments 

 which generally occur in distinct cells, so that the chemical 

 process is not initiated until, for some reason (e.g. injury), ferment 

 and glucoside come into contact. Thus, a glucoside amygdalin 

 occurs in the seeds of the Bitter Almond (but not in the culti- 

 vated form), whilst the appropriate ferment emulsin is situated 

 in the skin ; on crushing the seeds decomposition of the amygda- 

 lin into glucose, benzaldehyde, and prussic acid takes place, this 

 last being responsible for the poisonous properties. The hot taste 

 of many members of the Cruciferae (e.g. Horse Radish, Cress) is 

 due to the formation of mustard oil (together with glucose and 

 potassium hydrogen sulphate) by the action of a ferment (myrosin) 

 on another glucoside (sinigrin or myronate of potash). 



In the two instances just given the glucosides undoubtedly 

 render the plant distasteful to animals, but in many cases they 

 seem to serve as a means of storing glucose in a form which does 

 not diffuse readily. Thus the leaves of many Willows contain 

 a glucoside salicin which, during the night, is split up by the 

 enzyme salicase into glucose and saligenin ; the former is re- 

 moved, whilst the latter combines with the new sugar formed 

 the next day. To the glucosides also belong : the saponins found 

 in the Soapwort (Saponaria), Quillaia-bark, and many other 

 plants, and easily recognised by the formation of a froth when 

 shaken up with water ; the active principles of the Foxglove 

 (Digitalis) , the most important being digitalin, which has a pro- 

 found effect on the action of the heart ; and the indican of the 

 Woad (Isatis tinctoria) and of the Indigo-plant (Indigo/era] , the 

 latter being the source of natural indigo. 1 



1 Vanillin, the cause of the aroma of Vanilla (obtained from the pod 

 of Vanilla planifolia, a tropical Orchidaceous climber), is an aldehyde, 

 similar to those often combined with glucose to form glucosides. 



