Explain the discovery of plant growth regulators.

(N/A) The discovery of plant growth regulators (PGRs) is a result of various observations and experiments:
$1$. Auxins: Charles Darwin and his son Francis Darwin observed that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source (phototropism). This led to the isolation of Auxin by $F$.$W$. Went from tips of oat seedlings.
$2$. Gibberellins: $E$. Kurosawa reported the appearance of symptoms of 'bakanae' (foolish seedling) disease in rice seedlings caused by the fungus Gibberella fujikuroi. The active substance was later identified as Gibberellic Acid.
$3$. Cytokinins: $F$. Skoog and Miller identified that the cytokinin (kinetin) was a modified form of adenine,which promotes cell division in tobacco pith culture.
$4$. Ethylene: $E$. Cousins confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas.
$5$. Abscisic Acid $(ABA)$: Three independent researchers reported the purification and chemical characterization of three different kinds of inhibitors: Inhibitor-$B$,Abscisin $II$,and Dormin. All three were later proven to be chemically identical and named Abscisic Acid.