How were cytokinins discovered?

(N/A) $F. Skoog$ and his co-workers,while studying the nutritional requirements of tissue culture derived from the internodal segments of tobacco stems,observed that the internodal segments (a mass of undifferentiated cells called callus) proliferated only when the nutrient medium containing auxin was supplemented with extracts of vascular tissues,yeast,coconut milk (liquid endosperm of coconut),or $DNA$.
$\Rightarrow$ It was later found that the active substance was a modified form of adenine,which was crystallized and identified as kinetin.
Further,the compounds that exhibited kinetin-like properties were termed as cytokinins.