Norwegian physician, bryologist, botanist (1852–1917).
Ingebrigt Severin Hagen (1852- 8 June 1917) was a Norwegian physician and botanist who specialized in the taxonomy of the bryophytes. Born in Trondheim to shoemaker father Ingebrigt Hagen and Caroline Elizabeth née Helle, Hagen was academically gifted and graduated in arts from the Trondheim Cathedral School in 1870. He worked under Professor Worm Müller on physiological chemistry from around 1874–79 at Oslo. He then continued research in Uppsala, Sweden with Professor Holmgren in 1877 and then at Berlin, Leipzig and Dresden. He received a medical degree in 1883. He worked in Stören (1883–1884