ALKALOIDS 361 



Even this definition is not entirely satisfactory, as it would 

 include substances which, owing to their properties, could 

 hardly be classed as alkaloids, and excludes others, such, for 

 example, as hordenine. 



ALKALOIDS. 



Occurrence. 



The alkaloids do not appear to have a wide distribution in 

 the vegetable kingdom. Amongst the Angiosperms, the Apo- 

 cynaceae, Leguminosse, Papaveracese, Ranunculaceae, Rubi- 

 aceae and Solanaceae stand out in the provision of several of 

 these substances. The Labiatse, Rosaceae, Orchidaceae, and 

 Monocotyledons and Gymnosperms very rarely contain them. 



Alkaloids may occur in solution in the cell sap, especially 

 in young parenchyma : in older tissues the substances in 

 question may be stored in the solid state. They are found 

 in the seeds and fruits more particularly, but in the case of 

 the alkaloids of the Solanaceae and some other plants they 

 occur in the leaves, whilst the roots are the chief sources of 

 the alkaloids of Aconitum, Corydalis, and Hydrastis. The 

 cinchona alkaloids, and also pelletierine of the pomegranate, 

 are contained in the bark of their respective trees. 



With regard to their distribution in the different members 

 of the plant, there is so much variation that a single example 

 must serve. Stanek * found that the percentages of betaine, 

 expressed in terms of dry weight, occurring in Lycium har- 

 barum, were young leaves 3-91, old leaves 1-62, flowers without 

 calyx 1-5, young shoots 1-55, bark of root -49, and wood -12, 



Classification. 



The classification of the alkaloids is based upon the struc- 

 ture of the nucleus upon which their molecules are built up. 

 Five groups of alkaloids are accordingly recognized. 



I. Pyridine Alkaloids. — These, as the name implies, are 

 all derivatives of pyridine, and include — 



Coniine from Conium maculatum. 



Arecolin from Areca catechu. 



* Stanek : " Zeitsch. Zuckerind.," 1913. 37, 385. 



