PHENOLIC SUBSTANCES 209 



in the pathway the block to the synthesis of a secondary substance 

 occurs, the more hkehhood that the metaboHsm of a basic metabohte 

 is affected adversely. It follows that the early stage mutants would be 

 ehminated more often. Modification of a terminal step is also less 

 likely to provide the opportunity for the appearance of a new series 

 of compounds. The question of biochemical selection is also pertinent. 

 If selection becomes more critical, then the earlier in a sequence of re- 

 actions the change represented by the mutant occurs, the more likely is 

 its preservation to become dependent upon the total gene pool. Thus, in 

 general, the preservation of such a mutation rests upon a broader 

 underlying genomic constitution than that of a mutation affecting a 

 terminal step. Perhaps such considerations are purely academic at the 

 moment with respect to the systematic implications of biochemical 

 data, but they are nontheless potentially significant. Such considera- 

 tions bear upon the question of whether each newly acquired synthetic 

 ability should be given the same weight of systematic significance. In 

 the writers' opinion they should not. Even without consideration of 

 the actual systematic distribution of the compounds involved, the 

 appearance of an aurone (with a five-membered heterocyclic ring) 

 may be more significant than the appearance of a different glycosidic 

 pattern, although in the former case the empirical chemical formula 

 remains the same while in the latter it may be radically altered. 



In summary it is evident that knowledge of the major bio- 

 synthetic route and some familiarity with the mode of inheritance of 

 a group of related chemical constituents should allow more critical 

 analysis and a more precise evaluation of the systematic implications 

 of a given distributional pattern. 



Systematic aspects of the distribution of 

 phenolic compounds 



The use of phenolic substances in systematic investigations 

 does not extend back as far as that of certain other groups of plant 

 constituents such as alkaloids and essential oils. In the past decade 

 a number of investigators have considered the phenolics, particularly 

 anthocyanins, leucoanthocyanins, flavonols, and phenolic acids. Bate- 

 Smith has stressed especially the leucoanthocyanins. In his first treat- 

 ment of the systematic distribution of leucoanthocyanins (Bate-Smith 

 and Lerner, 1954), over 500 species were surveyed for leucoantho- 

 cyanins in leaves. In general these compounds are more abundant 

 in woody families, especially in certain groups regarded by some 



