Why do Leaves Fall in the Fall?

Why do Leaves Fall in the Fall?
Backyardnature.net

For those of us living where winters are severe, it's obvious that trees must lose their leaves during the fall, else those leaves will freeze and die when the first cold weather arrives. However, as you might suspect, there's more to the story than that...

In plants such as trees there are five chemical substances known as plant hormones. Three of these plant hormones promote plant growth and two inhibit it. The three growth promoters are auxins, gibberellins and cytokinins. The two growth inhibitors are ethylene and abscisic acid.

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