PHENOLIC SUBSTANCES 213 



contains the rather uncommon dihydrochalkone, phloridzin, as 

 its principal phenolic, while pear contains another uncommon 

 phenolic, arbutin, a glycoside of hydroquinone. Phloridzin is absent 

 from all pear species while arbutin is absent from apple. Even more 

 significant is the disclosure that, among the twenty-five species of 

 apple other than the cultivated apple (Malus pumila), in most species 

 phloridzin is the dominant phenolic, but in some species phloridzin is 

 reduced greatly in amount, and another dihydrochalkone glucoside 

 occurs, the second containing one more phenolic hydroxyl group and 

 with glucose attached at a different position than in phloridzin. The 

 second compound is found, with the exception of one variant of one 

 species, only in the four species from eastern Asia comprising the 

 series Sieboldianae. It is difficult to ignore the phyletic significance of 

 such data. 



Other examples of the potential value of rather unusual 

 phenolics which have a restricted distribution are the isoflavones and 

 the rotenoids. The former are reported only from the Rosaceae, 

 Leguminosae, Moraceae, and Iridaceae. In the first two instances, 

 since these families are closely related and often placed in the same 

 order, phyletic significance may be inferred while the other cases 

 doubtlessly represent convergent evolution. The rotenoids are, to the 

 writers' knowledge, restricted to the Leguminosae. The presence of 

 both isoflavones and rotenoids together is further circumstantial 

 evidence of a biosynthetic relationship between the two chemical 

 classes as suggested on chemical grounds earlier in this chapter. It 

 would be interesting to know if any species of Rosaceae produce 

 rotenoids. In general the phenolic chemistry of the Rosaceae and 

 Leguminosae are not similar (Bate-Smith, 1961). 



In the genus Iris the distribution of isoflavones appears to be 

 correlated with the morphological species groups delimited by taxon- 

 omists. Isoflavones are found only in the sections Evansia and 

 Pogoniris, considered as equivalent eastern and western groups. As 

 noted earlier, section Apogon contains most of the leucoanthocyanin- 

 positive species. Bate-Smith noted that leucoanthocyanins are gen- 

 erally found in the mesic species of Iris and that this generaliza- 

 tion seemed to apply to other monocots as weU. Recently, Reznik and 

 Neuhausel (1959) reported on the occurrence of colorless anthocyanins 

 in submerged aquatics. They found that a large number of such 

 aquatic species contained a high concentration of colorless antho- 

 cyanins, but these were not leucoanthocyanins. Rather, they were 

 presumed to be the pseudobase form of the anthocyanin which turns 

 red in HCl in the cold. True leucoanthocyanins must be heated in 

 rather concentrated HCl to produce corresponding anthocyanidins. 



