Career
He travelled extensively through Central and South America, and published a number of books on cacti, including the six-volume, 4,000-page Die Cactaceae, 1958-1962, and the Kakteenlexikon, first appearing in 1966 and updated posthumously. Although he collected and described many new species and defined a number of new genera, much of his work was based on faulty assumptions about the evolution of cacti and was too focused on geographic distribution. Many of his genera have since been reorganized or abandoned.