Morphological characters and statistical analyses
A total of 34 morphological characters were selected for the analyses
and scored for all specimens (Table S2). Characters were selected to
cover all parts of the plant generally available on herbarium specimens
(stem, leaves, calyces, flowers, and sexual organs) and care was taken
to avoid intrinsically dependent characters as well as characters
directly related to the size/vigour of the plant. Indumentum characters
are widely used, and generally considered important, in Menthataxonomy (Šarić-Kundalić et al., 2009; Yu et al., 2018) and ten
characters referring to various types of trichomes were therefore
included (Table S2). All characters were treated as continuous traits,
although some were scored as multiple categories (Table S2). Cases of
missing data were treated by mean value imputations. The morphometric
data was analyzed using Principal Component Analyses (PCA) conducted in
R v.3.6.3 (R Core Team, 2021) and sub-clusters of specimens were
visually identified. To illustrate and assess the importance of
indumentum characters, separate PCAs were conducted first excluding all
characters referring to the density or structure of trichomes and
secondly only including the characters of indumentum (Table S2). In
addition, detailed analyses of the distributions of indumentum
characters between different taxa were conducted.