HJORN, Oscar (yorn), Swedish naturalist, born in Bagnildstorp, Sweden, in 1741; died in Paris in 1792. He was a preceptor in the family of the Duke of Mirepois, and, owing to the protection of that nobleman, obtained from Louis XVI. in 1776 a mission to South America to study the flora of that country. He explored for ten years the vast regions included between the river Amazon and the river Plate amid dangers of all kinds, suffering great hardships and sometimes persecution from the Spanish and Portuguese authorities. Although he was kept a prisoner during 1780-'2 by the Guarani Indians, he formed an herbarium of 1,100 specimens, and, returning to Paris in 1776, published "Les legumineuses arborescentes de l'Amerique du Sud," a work which caused a sensation in scientific circles as the first of that kind ever published in Europe (Paris, 1787); a "Dictionnaire raisonne de l'histoire naturelle de l'Amerique du Sud" (1789); "Choix de memoires presentes a l'Acaddmie des sciences sur divers objets de l'histoire naturelle" (Paris, 1791); and "Dix ans dans l'Amerique du Sud" (3 vols., 1790). The herbarium of Hjorn forms a part of the collection of the Museum of natural history of Paris.