The subject of this thesis is a monograph of the genus Arctium, now including also five species formerly placed into Cousinia. Based on herbarium material and some field observations, this study presents new insights on three subjects: a) the generic delimitation of Arctium, limitation within the A. minus-complex, b) the species deand c) the occurrence of hybridisation. It soon became clear, rather surprisingly, that five species of Cousinia possess hooked involucral bracts like Arctium in its classical concept. The study became focussed then on the relation between Arctium in its classical delimilation and the large Asian genus Cousinia. From a macromorphological point of view the five ‘arctioid’ species of Cousinia should be placed in the extended genus Arctium. The pollen morphological study showed that most species of Cousinia have a pollen type which is different from the Arctium type. However, 30 species of Cousinia of the subgenera Cynaroides (all species and Hypacanthoides), including the five ‘arctioid’ species, share their pollen type with Arctium (see Chapter 7). In the phylogenetic analysis, including both macro- and pollen morphological characters, the arguments to rearrange Arctium against Cousinia got strong support. It was concluded that there were good reasons for a formal redescription of Arctium, now comprising 11 species belonging to 4 sections. Arctium section Arctium comprises all species of Arctium in its classical delimitation. The following 3 sections are new combinations: sect. Pseudarctium [with 3 species: A. (C.) amplissimum, A. (C.) pseudarctium (comb. nov.), and A. (C.) umbrosum], sect. Nanarctium [A. (C.) arctioides], and sect. Lappaceae [A. (C.) lappaceum]. Especially troublesome was the specific delimitation in the resulting in the delimitation of three species: A. minus-complex (see Chapter 11), A. atlanticum, A. minus and A. nemorosum; Arctium pubens and A. chaberti are treated as synonyms of A. minus. Within the classical delimitation of the genus three more species are recognized, A. lappa, A. palladini and A. tomentosum. species are all included in These six A. sect. Arctium. Regarding the question on hybridisation in the genus Arctium the following was observed. Contrary to the current opinion, hybrids appeared to be rare (see Chapter 12). They often have a higher percentage of abortive achenes than the parents. The hybrid between A. minus and A. palladini (A. x semiconstrictum hybr. nov.) is recorded for the first time. Copies will be sent after receipt of payment of Dfl. 45.00 (incl. VAT/BTW) into our — Postal giro account nr. 111768 to the Rijksherbarium/Hortus Botanicus, Leiden, The Netherlands; or by — International postal money-order; — Eurocard/Mastercard; — ABN-AMRO Bank, Leiden. Account No. of: RUL CS B.B.W.M. / RH and HB, Leiden, The Netherlands. Price if paid by bank, postal giro or cheque: Dfl. 45.00 + Dfl. 20.00 banking costs. Please refer to ‘Monograph of Arctium’.