Skoog and Miller's Tobacco pith callus experiment

In 1957 Skoog and Miller put forth the concept of hormonal control of organ formation. In this classic paper, they showed that the differentiation of roots and shoots in tobacco pith tissue cultures was a function of the auxin-cytokinin ratio, and that organ differentiation could be regulated by changing the relative concentrations of the two substances in the medium; high concentrations of auxin promoted rooting, whereas high levels of cytokinin supported shoot formation. At equal concentrations of auxin and cytokinin the tissue tended to grow in an unorganized fashion. This concept of hormonal regulation of organogenesis is now applicable to most plant species.

Skoog & Miller (1957)