GENERAL EVALUATION 343 



would be an exciting discovery. Only a few years ago, the presence of 

 such a common anthocyanin as cyanidin in a member of the 

 Chenopodiaceae would be of slight interest— now, the betacyanin 

 work (Chapter 14, p. 278) has placed the matter into a totally differ- 

 ent perspective. Again, the presence of cyanidin in general is of little 

 significance, but since the presence of C — C glycosides has been 

 known, a new C — C cyanidin glycoside would be of great interest. A 

 report of a biflavonyl in an angiosperm would be exciting or a rotenone 

 in a major plant group in which isoflavones were unknown. A 2' — OH 

 substituted flavonoid pigment is immediately of systematic interest. 

 Conversely, common phenolic acids such as caffeic or ferulic acids are 

 not normally of great systematic importance, but large amounts of 

 sinapic acid in a gymnosperm would be interesting to the systematist. 

 The examples above are stated with as much conviction as statements 

 bearing on the morphological features of a plant, and it indicates a 

 serious misunderstanding of the present situation if one should assume 

 that we do not have a background of knowledge through which we 

 can interpret new biochemical data in systematic terms. 



Earlier in this section, it was stated that in populations of 

 Tradescantia one might encounter numerous flower-color forms. This 

 phenomenon occurs in several species of the genus and, in every 

 species examined so far, the colors of the flowers rest upon the total 

 amount of pigment and the relative amounts of two anthocyanins, 

 cyanidin glycoside and delphinidin glycoside. In fact, all qualitative 

 color differences in these flowers appear to result from a minor bio- 

 chemical difference, namely, a single OH substitution. Blue-flowered 

 plants have mostly delphinidin glycoside, pink-flowered plants have 

 mostly cyanidin glycoside, and purple-flowered plants have a mixture 

 of the two. While the biochemical basis of flower color can be expressed 

 rather simply, the genetic basis of flower color in this genus seems to 

 be quite complex (Alston, unpublished). In contrast to other situa- 

 tions already cited in Chapter 11 (for example, Harborne, 1960b), a 

 single gene may govern a rather complex chemical difference in the 

 flavonoid pigments. It hardly needs to be emphasized that knowledge 

 of the genetic basis of a biochemical difference greatly increases the 

 possibility that the systematic significance of the biochemical differ- 

 ence can be determined. Since phylogenetic relationship is based on 

 evolutionary concepts which rest principally upon genetic mecha- 

 nisms, then all differences, whether biochemical or morphological, 

 ought to be expressed in genetic terms for maximal systematic utility. 

 Up to now, only a minute proportion of either biochemistry or 

 morphology is understandable in a genetic sense— biochemistry best 

 in the more fundamental reactions (that is, amino acid synthesis), 



