Trichome micromorphology in Alcea L. and allied genera (Malvaceae) and its systematic

Trichomes of 26 species of the genus Alcea were investigated using light
(LM) and scanning electron microscopy (SEM). The trichomes show a great
micromorphological variation, which provides interesting data for species
delimitation in Alcea. Two basic types of trichomes can be distinguished
in the genus Alcea and the allied genera: glandular and eglandular. The
glandular trichomes can in turn be subdivided into two subtypes: capitate
and clavate. The eglandular trichomes can be subdivided into five
subtypes: simple, fascicled, stellate, fascicled-stellate and
pluri-radiate. Characters of taxonomic interest are: trichome density
(glabrous to dense), number of arms per trichome, orientation relative to
the epidermal surface (appressed to erect) and presence/absence of stalk.
According to our results the species of Alcea can be divided into four
informal groups based on trichome types. Our results support the exclusion
of annual Althaea from the perennial ones and its close placement to
Malva. In addition, the close relationship between perennial Althaea and
basal Alcea lineages is supported by our trichome micro-morphological
investigation. Based on the evolutionary framework provided by recent
molecular phylogenetic investigations, following trends can be proposed in
Malva alliance: long and narrowly armed trichomes are primitive against
the short and thickly armed trichomes, dense indumentum coverage is
primitive against the moderately dense or glabrous ones, the presence of
simple hairs on stem (particularly on leaves) is more advanced against
their absence, spreading villous-stellate and fascicled trichomes are more
advanced against the appressed stellate ones and clavate trichomes, which
were found exclusively in few species of Alcea, should be considered as a
derived state against the capitate ones, and potentially provide a
synapomorphy for the crown group of Alcea, but this conclusion needs to be
tested by adding more species to trichome morphological analysis.