Summary The dissertation investigates Gomphrena L. and allied genera with morphological and molecular data in order to test the phylogenetic relationships and to better understand the evolutionary history of this linage. An emphasis is on the actual Gomphrena clade and as well the diversity in Bolivia. Gomphrena (Gomphrenoideae) is the largest genus of the Amaranthaceae with an estimated number of 120 species. The areas of main diversity of this genus comprise South America with about two thirds of the species, Central and North America (about 15%), and Australia (about 20%). This genus largely grows in tropical climates. Some of the species are used as medicinal plants, and a few are cultivated as ornamental plants exhibiting attractive pigmentation in their tepals. The first chapter presents the state of knowledge and a general introduction into Gomphrena. It is important to note that previous molecular phylogenetic studies revealed that Gomphrena in the widely used circumscription of Schinz (1934) is a polyphyletic group. These past studies included only few species of Gomphrena and distinguished two main clades. The first clade includes the type species of the genus, G. globosa L., and certain Australian and American species, plus the allied genera Gossypianthus Hook., Lithophila Sw., and Philoxerus R.Br. (= Blutaparon Raf.) with not fully understood relationships. This group of Gomphrena s. str. is here called “Gomphrena clade” whereas the second group that includes G. elegans Mart., G. mandonii R.E.Fr. and others is part of a lineage called Pfaffia clade (not studied here). In addition, the first chapter presents the complex taxonomic history of Gomphrena. The second chapter focuses on the morphological characteristics of the Gomphrena clade. As a result 22 vegetative and floral morphological characters were defined with their respective states and assessed for 27 representative samples. The photosynthesis type (absence or presence of C 4 photosynthesis) was coded in the form of a single trait including morpho-anatomical characteristics. For chapter three a combined data set of chloroplast sequences (matK-trnK + trnL-F + rpl16) of the Gomphrena clade (including the allied segregate genera) and outgroups from Gomphrenoideae was analyzed with parsimony (MP), maximum likelihood (ML) and Bayesian inference (BI) methods. The 22 morphological characters were optimized on the Bayesian Maximum clade credibility tree using Bayes Traits. The results show that most vegetative characters including the annual life form have evolved multiple times, whereas some floral character states were identified as synapomorphies. An important result reveals that many of the nine characters defined for the androecium, most of which were used historically for the pre- phylogenetic delimitation of Gomphrena and other genera, appear homoplastic. The presence of a fused stamen tube without stamen tube appendages (but with lateral filament appendages in some species) is confined to the Gomphrenoid clade (Gomphrena clade plus Froelichia, iv Xerosiphon, and the Pfaffia clade) whereas the Alternantheroid clade (here represented by Pedersenia) differs by the presence of stamen tube appendages. The important character is the difference between inner and outer tepals showing that all members of the newly found core C 4 Gomphrena clade have a difference in the size of the two inner tepals. To the contrary, all other species present five tepals of similar size. Another interesting result is the detection of pseudanthia, acting as a visual attractant for pollinators. The pseudanthial leaves are typical for a “Mostly Andean clade”. But pseudanthial leaves also were derived two more times independently in G. boliviana and allies and G. meyeniana and allied. The combined plastid tree also shows with high support that the species of Philoxerus, Lithophila and Gossypianthus are nested within the core C 4 Gomphrena clade. For this reason, it is proposed here to include the three genera into Gomphrena and a respective treatment is provided. The extended taxon sampling for matK-trnK (80 samples) and nrITS (82 samples) show that most of the major lineages are congruently resolved between plastid and nuclear data with one exception in the early branching G. mollis - rupestris clade (that is C3). It is sister to all C 4 species of Gomphrena in the nrITS tree whereas it appears as a second branch after the G. prostrata - Guilleminea clade in all plastid trees. Ancestral character state reconstruction shows that C 4 photosynthesis arose in the common ancestor of the Gomphrena clade plus Froelichia but reversed back to C 3 in the lineage of G. mollis and G. rupestris. The age of the crown group of the core C 4 Gomphrena clade plus Froelichia is inferred as 18 Ma (10.2-28.4, 95% HPD), which corresponds to the mid-Miocene climatic optimum (c. 18-16 Ma) when an increase in temperature and aridity and later drop in concentrations of CO 2 occurred. This result is also consistent with the emergence of C 4 photosynthesis in Chenopodiaceae and Poaceae. The core Gomphrena clade stands out by being a C 4 clade that diversified at least twice into high elevation Andean environments. Species such as G. fuscipellita (growing at elevations of 3600-4300 m) and G. meyeniana Walp. (3200-4700 m) constitute at the moment the highest populations of any herb C 4 . The mostly Andean clade diverged around 8 Ma (4.0-13.6, 95 % HPD) from lowland ancestors whereas the crown group has an age of just 4.3 Ma (1.8-10.3. 95% HPD). The majority of the species from the inter Andean dry valleys, Prepuna and Puna ecoregions in the extensively sampled matK-trnK and ITS trees are included in two major subclades of the “Mostly Andean clade”. The molecular time divergence estimate concurs with the maximum Andean mountain upheaval that occurred at 5 Ma, which created new habitats through the geomorphological and climatic modifications. The dry climates led to the recent and multiple evolution of annual species such as G. phaeotricha, G. pallida or G. umbellata. The sister of the Mostly Andean clade is the Australian clade., The results of this thesis (chapter 3) show with high support that the disjunctly distributed Australian species are closely related to the coastal species of Lithophila and Philoxerus. The latter is distributed on the Pacific coast of central America includes morphologically similar species that are endemics on the Galapagos islands. the same applies to Lithophila with one widely distributed Caribbean species and Galapagos endemics. The species of Lithophila and Philoxerus present adaptations to live in high concentrations of salt such as fleshy leaves, adventitious roots allowing vegetative reproduction from broken-off stems. The current distribution of Philoxerus vermicularis is restricted to the west coast of tropical Africa, and the species has not further extended into Africa, which decreases the possibilities for dispersal between Africa and Australia. The stem age of the clade comprising Lithophila, Philoxerus and the Australian Gomphrena spp. 10 Ma (5.7-16.9 95 % HPD). This confirms that the disjunction occurred recently so that Gondwanan vicariance is not possible a tectonic. It is therefore hypothesized that the dispersal to Australia involved long distance dispersal (LDD). Ancestors of Lithophila or Philoxerus -like plants came across the Pacific Ocean, perhaps even from Galapagos that acted as a stepping stone through marine currents such as the South Ecuatorial Current, which then leads over into the East Australian current and which were present since about 6 Ma. The chapter four provides a taxonomic treatment for the species of the core C 4 Gomphrena clade in Bolivia, including keys for identification detailed morphological descriptions and distribution maps. In the introductory part to this chapter dot distribution maps are presented, which are layed over by maps with environmental data in order to better standardize the habitat types of the species for the descriptions of their respective ecology. The results are based on extensive field and herbarium studies and now recognize 30 species of core Gomphrena in Bolivia, which is an increase in eight over the state of knowledge in the Catálogo de Plantas Vasculares de Bolivia that was published in 2014 with 22 species of core Gomphrena. These eight species represent discoveries new to science.