156 BIOCHEMICAL SYSTEMATICS 



not a chemically natural group, and it likewise follows that they do 

 not constitute a natural biological group, functionally, phylogeneti- 

 cally, or with respect to their biosynthesis. Therefore, few generaliza- 

 tions relevant to any of these above considerations are warranted. 

 Among nitrogenous substances of plants there is almost a continuum 

 from the universal products of metabolism to alkaloids in the strict 

 sense, and of course nitrogen-containing secondary compounds exist 

 which are not classified as alkaloids. Purine and pyrimidine bases and 

 the amino acid, histidine, are alkaloids except by the physiological 

 criterion. Betacyanins (formerly regarded as nitrogenous anthocyanins, 

 discussed in Chapter 14), except for the absence of any obvious 

 physiological effects, are clearly model alkaloids. 



Generalizations concerning the stability of alkaloids in the 

 plant, factors affecting their synthesis, origin within the plant, and 

 histological distribution must also be treated conservatively because 

 alkaloids comprise such a heterogeneous group. 



Since, by definition, alkaloids are physiologically active upon 

 animals, and many alkaloids are important drugs, the compounds are 

 best known to the pharmacologist. Much of the voluminous literature 

 on alkaloids is the direct or indirect result of their great economic im- 

 portance. It is probable that alkaloids are less well known to most 

 botanists than are certain compounds or classes of compounds that 

 serve some structural or obvious functional role in the plant (for ex- 

 ample, lignin, and plastid pigments). Consequently, a brief general 

 discussion, including a limited treatment of the chemical affinities of 

 the major classes of alkaloids will precede the section directly treat- 

 ing their sytematic significance. In this latter section no attempt is 

 made to give a comprehensive account of alkaloid distribution or to 

 develop any unified system of phylogenetic interpretation. Each of a 

 number of more natural classes of alkaloids could be given such a 

 treatment, and in fact some investigators have already done so. Cer- 

 tain of these latter types of studies will be described, but they have 

 been selected mainly to provide further insight into general principles 

 applicable to the evaluation of the systematic worth of alkaloids. 



In general, discussion of biosynthetic mechanisms past the 

 point required to clarify some point of phylogenetic interpretation is 

 beyond the scope of this book, especially in the case of the alkaloids, 

 wherein many classes of compounds exist, each of which may be 

 formed by almost completely independent biosynthetic routes. The 



1 According to Elderfield (1960): "No completely satisfactory all-inclusive defini- 

 tion of these compounds is possible. It will be sufficient to define an alkaloid as a nitro- 

 genous substance usually of plant origin, usually possessing basic properties, usually 

 optically active, and usually possessing some characteristic physiological action. Such a 

 definition is not perfect, and exceptions to all of the above criteria can be cited." 



