It was observed during the early 20th century that the abscission zone of leaves of tree growing close to gas burning street lamps weakened (entered the abscission phase) earlier than trees located...

Extracted text: It was observed during the early 20th century that the abscission zone of leaves of tree growing close to gas burning street lamps weakened (entered the abscission phase) earlier than trees located further away from the lamps. Experiments showed that weakening of the abscission zone was not caused by either the lamp's light or heat. Instead the research on this phenomenon led to the discovery of a new gaseous plant hormone. What is this hormone? Ethylene Abscisic acid (ABA) Gibberellins Auxin Strigolactone Cytokinin