Monograph of Arctium L. (Asteraceae). Generic delimitation (including Cousinia Cass, p.p.), revision of the species, pollen morphology, and hybrids

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, b) the species delimitation within the A. minus-complex, and 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 delimitation 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 (all species of the subgenera Cynaroides 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 A. minus-complex (see Chapter 11), resulting in the delimitation of three species: 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. These six species are all included in 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. xsemi-constrictum hybr. nov.) is recorded for the first time.