Thyme is an aromatic plant used both for medicinal purposes and as a spice almost everywhere in the world. A common feature of these plants is the presence of secretory hairs of different shapes, containing volatile oils. The purpose of this study was to describe comparatively the vegetative organs of the six species (less-investigated species) of seedling belonging to the Thymus genus.

For histo-anatomical investigations, plants belonging to six species of Thymus, species that grow in Romania, were obtained in laboratory conditions: Th. vulgaris, Th. serpyllum, Th. comptus, Th. praecox, Th. zygioides and Th. balcanus. In order to investigate the histo-anatomy, the plant material was fixed and preserved in 70% alcohol, and was then sectioned with the hand microtome, using the botanical razor. Obtained sections were coloured with green iodine and red ruthenium, and were then analysed with a Novex microscope and photographed. To investigate the secretory structures, we used SEM (Scanning electron microscopy).

The root generally presents a diarch-type structure, with the transition from the primary to the secondary structure being early. In the seedling Thymus balcanus, the root is much thinner, compared to the other species, with the rhizodermis cells varying in size; the bark is also thinner, presenting three layers of cortical parenchyma and an endodermal layer (in which Caspary thickening is very rarely observed). The hypocotyl has an intermediate structure between the root and the stem. The epicotyl presents a cauline structure with three distinct anatomical areas: the epidermis, bark and the central cylinder. In Thymus vulgaris at the level of the epicotyl, the bark is thinner, and the cells from the marrow are disorganized. Regarding the leaf, in Thymus praecox the palisadic tissue is much thicker than the other species. In all analyzed species, the presence of secretory hairs with uni-, bi- and octocellular glands was observed.