Monday, November 7, 2016

Leaves don't fall--they're pushed!

Red maples on West Elm Street, Brockton on 11/6/16--only a day or so after strong winds.

How else could you explain the piles of leaves now gathering, simultaneously, under so many trees?  It was very windy a day or two ago, yet only now, in stillness, are the leaves falling in large numbers. 

Many assume that in fall leaves die and fall off, while in fact the deciduous "habit" is an active process of gradually dismantling cellular machinery, retrieving some elements for recycling into next year's growth, and then cutting loose the "used up" leaf that remains. 

After the greater part of phosphorus and potassium and especially nitrogen are withdrawn from the leaf to be stored in trunk or roots, a special "breakaway" zone called the abscission zone develops at the base of the petiole (leaf stem).  Part of this zone includes a waterproof "bandaid" coated with fatty suberin that prevents "bleeding" from the wound that will result.  After this, the slightest breeze--or even just the weight of the leaf--will break it loose. 

View through a microscope.  The twig is on the left, the leaf grows out on the right.  Each little chamber you see is a single cell.  You can see how the leaf is being separated from the twig beginning at a sort of notch.

If you look at any twig you will see the leaf scar beneath each bud that marks where a leaf was attached.  Towards the middle of the leaf scar is one or more bundle scars that mark where the abscission zone cut through the vascular tissue that transports water and minerals upward (called xylem), and sugar downward (called phloem).  These scars vary in shape and arrangement from species to species, and are a good aid to identification.  

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