290 BIOCHEMICAL SYSTEMATICS 



approach to systematics is through surveys of antimicrobial or 

 antifungal agents of plant species. Nickell (1959) has summarized 

 much of these data listing the species and parts of those plants 

 tested, type of extract used, and the groups of organisms which were 

 affected. He also included a list of those species showing no activity, 

 though the latter does not include reference citations. A glance at the 

 Nickell summary is sufficient to indicate that numerous plant families 

 contain some species with antimicrobial compounds. Skinner (1955), 

 in his review of antibiotics in higher plants, compiled a list of the sur- 

 veys of plants for antibiotic activity. His compilation included refer- 

 ences to studies which surveyed up to 2,300 species representing 166 

 famihes (Osborn, 1943), as well as more limited surveys. In the 

 broader surveys specific inhibitory agents were not characterized, and 

 such studies are accordingly limited. However, a number of toxic 

 agents have been characterized, and these fall generally into one or 

 another of the classes of compounds which we have already taken up. 

 Table 14-4 lists the named antibiotics and one may recognize quinones 

 (2-methoxy-l,4-napthoquinone), flavonoids (quercetin), phenols (pro- 

 tocatechuic acid), alkaloids (berberine), and others. Also certain 

 essential oils, and even fatty acids (for example, linoleic) have been 

 reported to have some antibiotic activity. There are numerous anti- 

 biotics which do not fit readily into any of the major categories de- 

 scribed. These probably occur in a hmited number of species and, 

 therefore, represent further possibilities for systematic use. Such com- 

 pounds are first recognized by virtue of their antibiotic activities, we 

 may conclude that, although antibiotic activity per se is meaningless, 

 it represents a method of disclosing a new substance or class of sub- 

 stances which may prove to have systematic significance. For example, 

 species of Allium (onion) contain several antibiotic agents, some of 

 which are phenoHc, some of which are unidentified. GarUc {Allium 

 sativum), however, has been shown to contain a sulphur compound, 

 allicin, of the following chemical structure: 



CH2=CHCH2— S— S— CH2— CH=CH2 

 



allicin 



This compound does not readily fit any of the groups previ- 

 ously discussed. Allicin and various allyl sulphides are characteristic 

 of the genus Allium. 



Two other compounds which exhibit antibiotic activity and 

 which do not fit any of the biochemical categories previously covered 

 are protoanemonin and anemonin, from Anemone Pulsatilla: 



