Iltis spent nearly 40 years (1955 to 1993) as Botany professor and Director of the Herbarium at the UW-Madison, growing the herbarium to house over 1 million dried plant specimens. His taxonomic research focused on Capparidaceae and on Zea, working primarily in Mexico and the tropics. Iltis led numerous expeditions to many parts of the world to search for new plant species, travelling on mule or horseback when necessary.
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