The Taxonomic Relevance of Achenial Trichomes in Invasive European Asteraceae
Achenial trichomes of 23 taxa of four genera of Asteraceae were studied by means of digital and SEM: eight intrageneric taxa of Solidago L., one of Euthamia (Nutt.) Cass., seven of Erigeron sect. Conyza Less., and seven of Bidens L. It is proved that achenes of all the examined taxa of Erigeron sect. Conyza, Euthamia, and Solidago are covered with duplex trichomes. Within Bidens this feature is variable: indigenous European species B. tripartita L. and B. cernua L. are characterized by simple monostichous multicellular trichomes; the invasive B. frondosa L., by duplex trichomes; the alien B. connata Muhl. ex Willd., by trichomes of both types. Additional characters of taxonomic value were described for several taxa: trichome length, pubescence, surface sculpture, etc. Identification keys based on trichome characters were created for species of Bidens and Solidago.