124 THE PHYSIOLOGY OF PLANTS 



substances in the vegetable kingdom, which are split up by 

 acids and ferments into sugar and an indifferent substance. 

 An example of this group is amygdalin, a nitrogenous form 

 which occurs in the stone of various fruits (cherries, almonds, 

 &c.), which have the flavour of bitter almonds, due to oil of 

 almonds. This oil, as well as prussic acid and sugar, are 

 derived from the amygdalin by the action of the ferment called 

 emulsin. Another well-known substance is the oil of mustard ; 

 it is formed in the seeds of Cruciferag from sulphur-contain- 

 ing myric acid, a glucoside, which is split up by a ferment 

 {myrosin) into oil of mustard, sugar, and potassium sulphate. 



Very largely distributed in the vegetable kingdom, espe- 

 cially in young organs, are the tannin compounds, which are 

 chemically not very well defined. Of tannin itself we know 

 that it belongs to the glucosides. Allied to them are certain 

 colouring matters which are either dissolved in the cell sap or 

 occur in isolated drops. Other colouring matters ^ are also 

 devoid of nitrogen, soluble in alkalies and alcohol, but insoluble 

 in water. They occur only in the cell-walls, and are allied to 

 the resins which are rich in carbon, poor in oxygen, and devoid 

 of nitrogen, and which occur especially in the bud scales and in 

 woody tissues. Pure hydrocarbons occur in the form of etherial 

 oils, which in most cases give the plants their characteristic 

 odours. As an example we might mention the oil of camphor, 

 which yields camphor by oxidisation. 



The albuminous substances {proteids) are much less known 

 because they are less easily separated one from another, 

 and occur in numerous modification and transition stages. 

 Chemically we may distinguish, just as in the case of animal 

 albumen, three chief groups, the albumins, the caseins, and the 

 fibrins. Albumin is the name given to the albuminous sub- 

 stances which are soluble in the cell sap, but which coagulate 

 at a temperature of from 6o° to 70° C. Caseins are soluble in 

 water, do not coagulate on heating, but do so when treated 

 with acids. Fibrins are insoluble in water but partially soluble 

 in alcohol. The various members of these gi'oups often differ 

 considerably in theii' characters. Among the caseins, con- 

 glutin, which occurs in almonds, is much more soluble than 



^ Phlobaphenes. 



