Henk 't Hart (1944–2000)

Abstract

After being seriously ill for the last few months of his life, Henk 't Hart of the Population Genetics group at Utrecht University passed away on 22 Jul 2000. Henk aided enormously our understanding of the taxonomy and evolution of Crassulaceae. The study of this plant family, which had been the focus of his entire professional taxonomic career, started in 1978 with his Ph.D. thesis on the Sedum acre-group and Sedum series Rupestria. He collected Crassulaceae in Mexico and Northern America but mostly from the Mediterranean region, which he cultivated, and thus were permanently available for in-depth taxonomic studies. Cytological and morphological analyses of plants led him to describe several new species of Crassulaceae, some of which have not yet been cultivated in any other greenhouse.Using cultivated plant material, he provided clear evidence for the extraordinary degree of variation of chromosome numbers in many taxa of Crassulaceae.His numerous living collections have been the starting point for a series of projects, most of which were initiated during the early and middle 1990s. These provided unprecedented opportunities to study evolution of Crassulaceae because of their completeness and the possibility of using molecular markers. DNA markers for phylogenetic analyses were rapidly incorporated into research programmes by Henk and his collaborators. In fact, they were the first in The Netherlands to recognise the potential of DNA markers and to actually use these for phylogenetic systematic purposes. Among others, chloroplast DNA was used to examine relationships in Sedum series Rupestria, a series with a limited number of species but with 39 cytotypes. From the first successful applications of molecular data, Henk's interest for reconstructing the family's history using DNA markers strengthened.As methods of analysis and data collection became more advanced, the collections gained scientific value. Using strongly supported chloroplast DNA relationships in Crassulaceae as a basis, Henk initiated projects aimed not only at resolution of phylogenetic relationships, but also for using these phylogenies to reconstruct the evolution of independent characters. Examples of these reconstructions include, among others, evolution of chromosome numbers in Sedum series Rupestria, growth-form evolution and patterns of migration in Aeonium, the evolution of dysploid chromosome numbers in Sedum series Aithales, and history of secondary plant metabolite accumulation in Sedum, Aeonium and Sempervivum.Recently, Henk was able to demonstrate the starting point and path of intercontinental migration routes in North American Crassulaceae using seeddispersed molecular markers. He further demonstrated that these long distance dispersals in Northern hemisphere Crassulaceae were rare, but not unique.It was only after completion of the above-mentioned studies that it became possible to assess the impact of the completeness of the collection of species brought