Having befriended Alexander von Humboldt at Corvisart's house, he joined him on a five-year journey to Tenerife and the Spanish colonial empire in the Americas, traveling to what later became the independent states of Venezuela, Cuba, Colombia, Ecuador, Peru, and Mexico, as well as the Orinoco and Amazon basins, with a last stop in the United States. During this trip, he collected and classified about 6,000 plants that were mostly unknown in Europe up to that time. His account of these findings was published as a series of volumes from 1808 to 1816 entitled "Equatorial Plants" ().