Trichome morphology in <i>Teucrium</i> L. (Labiatae). A taxonomic review

Abstract

The micromorphology of trichomes of 56 Teucrium L. species belonging to the 9 sections of the genus in the Mediterranean area was surveyed by scanning electron microscopy (SEM) of leaves, calyx, corolla and nutlets. 25 trichome types are described, 12 of them are new. Thin walled hairs are the exclusive type found in the corolla and are the most widespread type on the abaxial side of the leaves. Subsessile glandular hairs, 2-4-celled, are found on the nutlet and leaves of the semi-shrubby and paleoendemic species. The presence of short or elongated, generally adpressed simple thick-walled slighuy conical hairs provides an additional character to clarify the boundaries between sect. Chamedrys (Mill.) Schreb, and sect. Polium (Mill.) Schreb. Branched non-glandular hairs are confined to sect. Polium subsect Polium , except for the rare branched hair conical and thick-walled type found in Teucrium barbarum Jahand. & Maire (sect. Chamaedrys ) and T. heterophyllum L`Her. from sect. Teucrium . This last section is well defined by the absence of simple slighuy conical thick-walled hairs and the glandular hairs in the corolla. Sect. Teucriopsis Benth, is of particular interest for the exclusive presence of branched and peltate glandular hairs on the nutlets surface. Sect. Chamedrys is a homogeneous group, distinguished from the other sections by an indumentum formed only by trichomes types evolved from the simple slighuy conical thick-walled hairs. This section overlaps, in the presence of glandular sub-sessile hair on the nutlets surface, with sects. Isotriodon Boiss, and sect. Polium subsect. Rotundifolia Cohen ex Valdes Berm. & Sanchez Crespo. The trichomes type of the calyx teeth, abaxial side of the leaf and latero-posterior corolla lobes can be used as a distinctive taxonomic character at specific and infra-specific level. This study supports Bentham's delimitauon of sections with the addiuons subequently made by Boissier.