280 



BIOCHEMICAL SYSTEMATICS 



are common in seed plants. All groups of the gymnosperms, except the 

 Gnetales, contain some tannin producers. Among angiosperms, the 

 monocot families Palmae, Musaceae, and Iridaceae are notably 

 tanniniferous. Dicot families of the orders Fagales, Rosales, and 

 Myrtales are particularly rich in tannins. They are rare or absent 

 in the families Gramineae, Caryophyllaceae, Cruciferae, Cacta- 

 ceae, Chenopodiaceae, Labiatae, Umbelliferae, and Primulaceae 



(Skene, 1934). 



Tannins are probably best considered as phenolics. In fact the 

 non-hydrolyzable, condensed tannins are flavonoid derivatives. These 

 are complex polymers which may form insoluble products (often 

 called phlobaphenes). Famihar examples of condensed tannins are 

 derivatives of catechin or gallocatechin. Their relationship to antho- 

 cyanins and leucoanthocyanins is obvious. 



OH 



OH 



HO 



OH 



HO 



OH 



OH OH 



HO 



catechin 



gallocatechin 



The hydrolyzable tannins may occur as glycosides— the agly- 

 cone often being a phenohc acid such as gallic acid. Brief but concise 

 recent reviews of tannin chemistry are those of Mayer (1958) and 

 Schmidt (1955), although these reviewers did not treat at all the sys- 

 tematic distribution. As noted in an earlier section of this chapter 

 Gibbs has studied the distribution of catechol tannins in the plant 

 kingdom without, however, deriving systematic patterns of any great 

 importance. 



Lignin: Lignin is a plant product which potentially is of great 

 systematic value, especially if technical advances occur which provide 

 a method of analysing the sequential linkages of the building 

 units and their cross hnkages. When Freudenberg (1959a) can raise 

 even a rhetorical question such as whether lignin is a "molecular com- 

 postheap" or consists of an orderly structure hke cellulose, one clearly 

 recognizes the present Umitations of our knowledge of hgnin. Even 

 the definition of lignin is based entirely on its degradation properties: 

 "That plant component which, when refluxed with ethanol in the 

 presence of catalytic amounts of hydrogen chloride, gives a mixture 

 of ethanolysis products such as a-ethoxypropioguaiacone, vanillin, 

 and vanilloyl methyl ketone from coniferous woods, and, in addition, 

 the corresponding syringyl derivatives from deciduous woods." 

 (Brauns and Brauns, 1960) 



