Thymus vulgaris has a chemical polymorphism with six different chemotypes that show marked spatial segregation in nature. Although some populations have a single chemotype in majority, many have two or three chemotypes. In this study we analyze the quantitative variation among T. vulgaris populations in the percentage of oil composed of the dominant monoterpene(s) for each chemotype. In general, phenolic chemotypes (thymol and carvacrol), which occur at the end of the biosynthetic chain, have a significantly lower proportion of their oil composed of their dominant monoterpene than nonphenolic chemotypes (geraniol, alpha-terpineol, and linalool). This is due to the presence of high amounts of precursors (gamma-terpinene and paracymene) in the oil of phenolic chemotypes. The essential oil of the nonphenolic thuyanol chemotype has four characteristic monoterpenes that together make up a lower proportion of the oil than the single dominant monoterpene of the other nonphenolic chemotypes. For all chemotypes, the percentage composition of the dominant monoterpene decreased significantly at sites where the chemotype is not the majority type. This decrease is correlated with a significant increase in either the proportion of the two precursors for the thymol chemotype or the monoterpenes characteristic of the other chemotypes at the site. The latter result suggests that a plant with dominant genes is responsible for the production of different monoterpenes can produce several molecules.

The chemical compositions of 15 commercial samples of essential oils of leaves of Myrtus communis L. from five different Mediterranean locations (Corsica, Morocco, Tunisia, Lebanon, and former Yugoslavia) were studied. Forty-seven compounds were identified by GUMS. They could be divided into two groups on the basis of their percentages of a-pinene; over 50% (Corsica and Tunisia), and under 35% (the others). The percentages of limonene, 1,8-cineole and myrtenyl acetate were consistent with this classification. The chemical compositions of these oils can be compared to those of Spanish oils.

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