GLANDULAR HAIRS, NON-GLANDULAR HAIRS, AND ESSENTIAL OILS IN THE WINTER AND SUMMER LEAVES OF THE SEASONALLY DIMORPHIC THYMUS SIBTHORPII (LAMIACEAE)

The structure and function of the glandular and non-glandular hairs, and also the yield and chemical composition of the essential oils in the winter and summer leaves of the seasonally dimorphic plant Thymus sibthorpii were studied. Glandular hairs comprise peltate hairs only (capitate hairs are missing). Peltate hairs are the sites of essential oil biosynthesis. They are more numerous in the winter leaves than in the summer leaves and consist of a 12-celled secretory head, a unicellular stalk, and an also unicellular epidermal foot. The essential oil of the winter leaves is mainly composed of linalool (42.4%), thymol (7.0%), p-cymene (5.8%), β-caryophyllene (5.7%), borneol (5.6%), and terpinen-4-ol (4.8%). The oil of the summer leaves is principally constituted of p-cymene (25.0%), linalool (19.1%), terpinen-4-ol (8.5%) and borneol (8.3%). Non-glandular hairs proliferate in the summer leaves. They are conical in shape and consist of one basal epidermal cell and one apical pointed cell. Glandular and non-glandular hairs are implicated in the chemical and mechanical defense of the plant, respectively.