Ostenf.

Carl Emil Hansen Ostenfeld (born Carl Emil Ostenfeld-Hansen) (3 August 1873 – 16 January 1931) was a Danish systematic botanist. He graduated from the University of Copenhagen under professor Eugenius Warming. He was a keeper at the Botanical Museum 1900-1918, when he became professor of botany at the Royal Veterinary and Agricultural University. In 1923, by the early retirement of Raunkiær's, Ostenfeld became professor of botany at the University of Copenhagen and director of the Copenhagen Botanical Garden, both positions held until his death in 1931. He was a member of the Royal Danish Academy of Sciences and Letters and served on the board of directors of the Carlsberg Foundation.

Ostenfeld is known as an explorer of the Danish flora, including marine plankton, as well as the flora of Western Australia.

Ostenfeld participated in the International Phytogeographic Excursion to the British Isles in 1911. The party studied the flora of parts of Ireland, including Killarney, Connemara and The Burren.

In collaboration with O. Rosenberg, he was one of the first to confirm that some plants could form asexual seeds, now called (apomixis). Their experiments repeated those of Gregor Mendel with Hieracium hybrids, showing that Mendel had observed a mixture of sexual recombination and apomixis.

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