Who first demonstrated crossover of homologous chromosomes in corn plants, and when?

1. Crossing over remained an abstract idea for two decades after Janssen saw chiasmata. 2. In 1931 Barbara McClintock and Harriet Creighton demonstrated it in corn plants. 3. Weeks later Curt Stern demonstrated homologous recombination in Drosophila microscopically. 4. Stern therefore followed the corn work rather than preceding it. 5. Sutton and Boveri had proposed the Chromosomal Theory nearly thirty years earlier. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 13 "Modern Understandings of Inheritance", section 13.1 Chromosomal Theory and Genetic Linkage_