Trichome morphology and its significance in the systematics of Phlomoides (Lamiaceae; Lamioideae; Phlomideae)

Abstract

Trichomes of 32 species of the genus Phlomoides representing its various natural groups were examined using scanning electron (SEM) and light microscopy (LM). Trichomes varied among different species of this genus but were identical among different populations of a certain species, and therefore provide valuable characteristics in delimitation of sections and species. The indumentum might bear the following trichomes alone or a mixture of them: glandular, non-glandular, branched, simple, papillate, and/or smooth on surface. Two basic types of trichomes can be identified: non-glandular and glandular. Both trichome types can be simple or divided. According to their size, simple non-glandular trichomes can in turn be subdivided into three subtypes: short, long and extremely long (longer than 3mm). Variation in trichome characters appears to have particular value, not only in classification at sectional rank, but also in separating different species of Phlomoides from their relatives. For example, most species of P. sect. Filipendula are characterized by dense extremely long or lanate trichomes. Moreover, P. tuberosa is unique among all studied species by having branched non-glandular trichomes papillate on surface. This result indicates the presence of branched trichomes with papillate surface as characteristic features of indumentum in the type species of this section. The following evolutionary trends are suggested here regarding trichome characters for Phlomoides: (1) multi-nodal branched trichomes are primitive against branched trichomes, (2) extremely long trichomes are primitive against the long or short simple trichomes, and (3) loss of glandular trichomes is advanced against their presence.