Plant geography seeks to describe the patterns of species distributions and understand the underlying mechanisms. The foundation of the field is attributed to Alexander von Humboldt following the broad-scale insights he gained on his explorations of the New World two centuries ago. Today, in the age of “Big Data,” advances in methods and data availability allow us to better assess the complex drivers of species distributions. In this essay, we give an overview of the relevant legacy of Humboldt in the field of plant geography. Comparing the foremost insights and approaches of Humboldt's time and of today, we highlight areas in which major changes have taken place and areas in which Humboldt's approach is still relevant. We present advances in the description and understanding of plant geography, which have changed our entire worldview to a much more dynamic one. Further, we present some of the outstanding challenges of the field, and how solving them requires going back to the “Humboldtian” approach to science, i.e., combining precise, quantitative empirical studies with a holistic approach. Finally, we discuss how our recognition of the planetary impact of humans in the Anthropocene means that much of our research is no longer exclusively driven by curiosity, but also by the societal need to make predictions of ecosystem responses to environmental change. There is thus a strong need for combining quantitative empiricism with a holistic perspective to address the complex challenges of the current epoch.

Ongoing studies of Neotropical Euphorbiaceae have uncovered a surprising number of new species belonging to Croton L. sect. Cleodora (Klotzsch) Baill., one of the better studied clades in the genus. Here we describe six new species and reclassify three others that were either misplaced to section or regarded as species of unknown phylogenetic affinity in previous studies. The new species are all from South America, and five of them are restricted to the Amazon region. We update the most recent taxonomic revision of Croton sect. Cleodora by providing a new taxon list and an identification key for the 27 currently recognized species. Sixteen species occur in lowland forested areas of the Amazon basin. Our finding calls attention to the Neotropics, and the Amazon region in particular, where information about the flora and species geographic ranges is still incomplete, even for relatively well-studied plant groups.

A taxonomic revision of Trisetum Pers. sect. Trisetum is presented. We include descriptions and synonyms of each taxon from a study of 894 vouchers from 45 herbaria. Detailed morphometric descriptions, illustrations, distribution maps, an identification key, and habitat data are given for each taxon. Morphometric variation of the main characters is shown by box plots. Thirty-one names are lectotypified. Two neotypes are designated. We recognize eight species of Trisetum in the section: T. alpestre (Host) P. Beauv., T. altaicum Stephan ex Roshev., T. bertolonii Jonsell, T. flavescens (L.) P. Beauv., T. fuscum Schult., T. glaciale (Bory) Boiss., T. gracile (Moris) Boiss., and T. laconicum Boiss. & Orph. Two infraspecific taxa of T. flavescens are recognized (T. flavescens subsp. flavescens and T. flavescens subsp. griseovirens (H. Lindb.) Dobignard). Six of the eight species of Trisetum sect. Trisetum are endemic to the different European mountain ranges, while T. altaicum grows in the Altai and Tian Shan Mountains, and in the mountains of northern Mongolia and southern Russia, and T. flavescens is widespread in temperate regions of Europe, western Asia, and northern Africa. Vegetative propagation by pseudoviviparism is observed for the first time in specimens of T. flavescens subsp. flavescens.

Analyzing pollen morphological data on a contemporary phylogenetic framework can enhance our understanding of the distribution, diversity, and evolution of palynological characters. In this paper, the sixth in a series detailing pollen morphological characters across angiosperms, we focus on the Celastrales, Oxalidales, and Malpighiales (COM) clade and Zygophyllales, together comprising ca. 20,000 species in 47 families within fabids. We first examined pollen grains from 21 species with light, scanning electron, and transmission electron microscopy, to illustrate pollen diversity within the COM clade and Zygophyllales. Second, based on a reexamination of previously published pollen data and our new observations, we optimized 21 pollen characters on a maximum likelihood (ML) tree of 169 genera in 45 families representing all four orders, using Fitch parsimony, ML, and hierarchical Bayesian inference. The pollen morphology of this group displays great diversity, particularly in size, aperture number, supratectal element shape, and tectum sculpture. Plesiomorphic states for 18 characters were inferred unambiguously under all methods for the COM clade, and over 300 character state changes were hypothesized by each analysis on lineages at different levels within the group. Changes of state were found to occur most frequently in the characters outline in polar view, pollen size, and tectum sculpture; changes of state occurred least frequently in the characters dispersal unit, symmetry, and tectum presence/absence. We identified diagnostic character states for several monophyletic clades and explored palynological evidence to shed light on some unresolved relationships. For example, the previously poorly resolved Malpighiales were found to be distinguished by a single pollen character state change (from annulus absent to present). Patterns of evolutionary change in several notable pollen characters, such as the number of pollen apertures (from three to other states) and tectum sculpture (from perforate to reticulate), were further confirmed, and their possible adaptive functions are proposed. Reflecting current interest in the relationship between pollen morphology and pollination syndrome, we conducted tests of correlated evolution between pollen morphology and pollination syndrome, with results demonstrating significant correlations between a thin exine and anemophily and, unexpectedly, between presence of supratectal elements and anemophily. Evidence for previously postulated evolutionary trends in aperture number (increasing), tectum sculpture (from perforate to reticulate), and infratectum structure (from columellate to granulate) is discussed. The relatively high frequency of state changes in presence or absence of supratectal elements may be linked to switches in pollination syndrome within this group.

All Onagraceae for which data are available have vestured pits on vessel-to-vessel pit pairs. Vestures may also be present in some species on the vessel side of vessel-to-ray pit pairs. Herbaceous Onagraceae do not have fewer vestures, although woods with lower density (Circaea L. and Oenothera L.) have fewer vestures. Some Onagraceae from drier areas tend to have smaller vessel pits, and on that account may have fewer vestures (Epilobium L. and Megacorax S. González & W. L. Wagner). Pit apertures as seen on the lumen side of vessel walls are elliptical, occasionally oval, throughout the family. Vestures are predominantly attached to pit aperture margins. As seen from the outer surfaces of vessels, vestures may extend across the pit cavities. Vestures are usually absent or smaller on the distal portions of pit borders (except for Ludwigia L., which grows consistently in wet areas). Distinctive vesture patterns were observed in the several species of Lopezia Cav. and in Xylonagra Donn. Sm. & Rose. Vestures spread onto the lumen-facing vessel walls of Ludwigia octovalvis (Jacq.) P. H. Raven. Although the genera are presented here in the sequence of a recent molecular phylogeny of Onagraceae, ecology and growth forms are more important than evolutionary relationships with respect to abundance, degree of grouping, and morphology of vestured pits. Designation of vesture types is not warranted based on the distribution of named types in Onagraceae and descriptive adjectives seem more useful, although more data on vesturing in the family are needed before patterns of diversity and their extent can be fully ascertained. Vestures are less common and may have a more complicated genetic basis than helical thickenings in vessels, but may be a more effective form of water column maintenance.