AMINO ACIDS 109 



pool. A generation of studies of genetic control by biosynthesis has in 

 fact aheady established this principle. 



Fowden and Steward concluded from their study that numer- 

 ous metabolic pathways, previously unexpected, existed. This conclu- 

 sion certainly appears to be valid, since many yet unidentified 

 compounds exist, and these offer further promise for comparative 

 biochemical studies. 



Birdsong et al. (1960) have reported on the distribution of the 

 guanidine, canavanine, an amino acid found thus far only in the 

 family Leguminosae. Within the family its appearance seems to be of 

 definite taxonomic significance. 



NH 

 ONHCNH2 



i 



H2 

 CH2 

 CHNH2 

 COOH 



canavanine 



Prior to this study a total of sixty-eight species representing 

 thirty-one genera had been analyzed by various workers, and 

 Tschiersch (1959) was of the opinion that since canavanine appeared 

 somewhat randomly in the family, its distribution had no taxonomic 

 significance. Extension of the number of investigated species disclosed, 

 however, that canavanine occurs only in the sub-family Papilionoi- 

 deae, and of the tribes of that sub-family, it does not occur in Pody- 

 larieae and Sophoreae and is apparently rare in the tribe Genisteae. 

 Canavanine is particularly common in the tribes Trifoheae and 

 Loteae; all of seventeen species in these two tribes analyzed by Bird- 

 song et al. contained canavanine. Przybylska and Hurich (1960) have 

 reported the canavanine distribution in a few additional species, but 

 the pattern of distribution is maintained. The lack of canavanine in 

 the tribes Podylarieae and Sophoreae is interesting because inde- 

 pendent chromosomal evidence suggests that these tribes are offshoots 

 from the main Papilionoid stock (Turner and Fearing, 1959). 



There is considerable circumstantial evidence that canavanine 

 is an important metabolite in those plants in which it occurs; for 

 example, it may be important in the storage and transport of nitrogen. 

 If this is true, the distribution of canavanine should be more vigorously 

 controlled by selection pressure, and therefore its distribution should 

 have greater significance (that is, subsequent loss of ability to form 

 canavanine would have negative survival value). The Birdsong et al. 



