Autumn, is often call “FALL.” because it is a season when many deciduous trees drop their leaves.
What causes these leaves to fall?
In deciduous trees, an abscission zone (a separation zone) is formed at the base of the petiole (stalk that attaches leaf to plant).
The separation zone has a top layer of cells with weak walls and a bottom layer of cells that expand in the autumn. The lower expanding cells cause the weak cells walls of the upper cell layer to break. This results in the leaf falling off the tree. (This is just a small part of the changes involved.)
OK. Now we have a clue about why the leaves break away, but what causes the bottom layer to expand? What starts the breaking process? Why does it happen in autumn?
The answer is simple but the process is very complex.
Because the days get shorter and cooler in the autumn, the amount of certain plant hormones change.Hormones are chemicals in living organisms that direct different processes, such as the expanding of the separation zone cells in deciduous trees.
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