TERPENOIDS 233 



It is evident that a very large number of simple terpene types are 

 theoretically possible (and, in fact, exist). More complex terpenoid 

 compounds, and other types of isoprenoid derivatives, also exist 

 in abundance throughout the plant kingdom and to a more limited 

 extent among animals. Even in simple monoterpenes internal rings may 

 form with the elimination of a double bond, for example, as in pinene. 



«-pinene 



Also, additional isoprene units may be incorporated. Ses- 

 quiterpenes, for instance, represent three isoprene units (C15 com- 

 pounds), and diterpenes represent four isoprene units. The latter are 

 relatively uncommon. Ti'iterpenes, with six isoprene units, are but 

 rarely encountered in higher plants. Plant steroids are best considered 

 allied with the terpenoid substances. Isoprene derivatives, or com- 

 pounds that may be derived theoretically from isoprene, of even 

 higher molecular weight include such compounds as the carotenoids, 

 and high polymers such as rubber or gutta percha. Although the 

 carotenoids may prove to be a valuable biochemical category for 

 chemosystematic purposes, as yet Httle work along such hues has 

 been done with such compounds except among the algae (Chapter 

 14). The phytyl group of chlorophyll is essentially a poly isoprene, and 

 the group also occurs as part of the napthoquinone derivative, 

 Vitamin K, discussed in Chapter 12. Such substances as these last are 

 examples of important basic metabolic pathways and are therefore 

 probably less useful in phytosystematic investigations. 



A classic example of the apphcation of phytochemistry to 

 problems of phylogeny is the work of Baker and Smith (1920) on the 

 terpenes of Eucalyptus oils. One might suppose that the impetus from 

 this classic work would have encouraged considerable interest in the 

 systematic distribution of terpenes, yet this has not occurred on a large 

 scale. The Baker and Smith work was a remarkable achievement, but 

 on reading the book it is nontheless evident that the immediate 

 systematic imphcations of the study were quite limited. There is no 

 doubt however that the work disclosed clearly the possibihties of 

 phytochemical systematics. Some of the important aspects of the 

 Baker and Smith work will be considered at this time. 



One important goal of their early investigations was to deter- 

 mine whether or not chemical characters are dependable (or constant) 

 enough to warrant their consideration as taxonomic characters. The 

 examination of large numbers of individual trees over an extensive 



