Karl Wilhelm von Nägeli (March 27, 1817 - May 11, 1891) was a Swiss botanist. He discovered what would later become known as chromosomes and apparently discouraged Gregor Mendel from further work on genetics.

Academic career
Among his more important contributions to science were a series of papers in the Zeitschrift fur wissenschaftliche Botanik (1844 1846); Die neueren Algensysteme (1847); Gattungen einzelliger Algen (1849); Pflanzenphysiologische Untersuchungen (1855 1858), with Carl Eduard Cramer; Beiträge zur wissenschaftlichen Botanik (1858-1868); a number of papers contributed to the Royal Bavarian Academy of Sciences, forming three volumes of Botanische Mitteilungen (1861-1881); and, finally, his volume, Mechanisch-physiologische Theorie der Abstammungslehre[scanned source], published in 1884. However, perhaps Nägeli is best known nowadays for his unproductive correspondence (1866-1873) with Gregor Mendel concerning the latter's celebrated work on Pisum sativum, the garden pea.
The standard botanical author abbreviation Nägeli is applied to species he described.