288 BIOCHEMICAL SYSTEMATICS 



distribution of isothiocyanates within the Cruciferae. Although much 

 information has been accumulated concerning isothiocyanates of this 

 family, only a few primarily systematically oriented studies have been 

 published (for example, Kjaer and Hansen, 1958). Certainly the iso- 

 thiocyanates represent a major source of taxonomic information in 

 the Cruciferae, and since isothiocyanates can now be analyzed by 

 paper chromatography, more studies of natural populations and their 

 isothiocyanates content should be forthcoming. 



Organic Acids: Although there are numerous organic acids 

 found in almost all plants, most of them have either been considered 

 elsewhere or they are so generally distributed that they offer no 

 great utility to systematics. In the former category are the "phe- 

 nolic" acids (for example, chlorogenic, cinnamic, coumaric, caffeic, 

 and numerous others). Not all of these acids are phenols, but they are 

 biosynthetically related to phenols and thus fit that category naturally. 

 These acids have been discussed in Chapter 11. 



Among the acids which are of such general distribution as to 

 be of no great value in systematics are oxalic acid, lactic acid, and all 

 those organic acids represented in the metabolic pathway leading to 

 the oxidative breakdown of carbohydrate. 



From the older literature the acids hydroxy-citric and a-hy- 

 droxyglutaric acids have been described from beet juice (Buch, 1957). 

 Towers and Steward (1954) found evidence of the presence of a-keto- 

 y-methylene-glutaric acid in tulip leaves. In fact it was the most con- 

 spicuous keto acid constituent. This acid is an analog of the amino acid, 

 y-methyleneglutamic acid, also found in all Tulipa species examined 

 by Fowden and Steward (1957). This has been discussed in Chapter 

 6, and the significance of the presence of the keto acid in Tulipa 

 is best related to the metabolism of the corresponding amino acids. 

 Furthermore, in the peanut {Arachis hypogaea), which also produces 

 certain of the uncommon glutamic acid derivatives, Fowden and 

 Webb (1955) detected y-methylene-a-ketoglutaric acid in the seedlings. 



Another example of considerable interest is malonic acid. This 

 acid, a competitive inhibitor of the Krebs cycle enzyme, succinic de- 

 hydrogenase, would not normally be anticipated in large quantities, 

 yet it has been reported to accumulate in the leaves of eighteen of 

 twenty-seven species of the family Leguminosae, in some species of 

 Umbelliferae, and elsewhere (Bentley, 1952). It is difficult to account 

 for the wide occurrence of this acid, and it presents an intriguing 

 physiological-biochemical problem. 



Stafford (1959, 1961) has studied the accumulation of tartaric 

 acid in angiosperm leaves and finds that, although many families con- 

 tain some species which accumulate the acid, it is found consistently 



