Why leaves fall off

As I drove about, enjoying the scientific understanding of why leaves turn their magnificent colors (see Why Leaves Turn Color) I then logically went to the next phase of the fall foliage process and began wondering to myself, ‘self? why do leaves fall?’ Why can’t these beautiful leaves that so romantically depict the coming of winter continue to remain throughout the year? Why don’t the strong connections that bind them to their branches remain during the cold, harsh winter months?

Launching an investigation, I turned to one of my favorite and most wisest companions, Bill Nye the Science Guy. He so effectively explained to me that, when fall comes around and the leaves turn their colors, “the air gets colder–often much colder. Along with cold air comes dry air. Cold air cannot hold as many water molecules per cubic meter or cubic foot as warm air.”

“Interesting Bill, but that still doesn’t answer my question,” I replied with confusion, one eyebrow raised.

He decided to take the scientific high road and told me, “The process of a deciduous tree closing up its food-making shop for the winter is chemically complicated. When the days get darker, plants have complex biochemistry that sets a series of events in motion. As the amount of light per day goes down, photoperiodic plants (deciduous trees in this case) produce ethylene and a substance called abscisic acid. That leads to a change in the ratio of the hormones auxin and cytokinin. Scientists named these compounds by their functions. Auxin is the hormone that makes cells elongate or grow, similar to the word augment. Cytokinin comes from words that mean “put division in motion.” When cells divide, they’re growing. As the ratios of these specialized plant hormones change, trees stop making the famous energy-producing chemical chlorophyll. They pull in all the nutrients they can from the leaves, and cut the leaves off from their main stems.”

Confused by all the big words and phrases, I managed to pull out a simple question: “Okay, so that means that since the ‘energy-producing chlorophyll’ is eliminated, the leaves not only turn color, but are cut off from the energy system of the tree and cannot stay connected to the stems, leaving them to ‘hang loose’ until the storms and winds of winter whisk them off the stem and onto the ground?”

“Precisely!” He exclaimed in his nerdy voice. “Deciduous trees developed a way to make hay–well, plant cells and plant sugars–only while the air is moist. Then these big, strong wooden plants found efficiency in abscising (cutting off) their leafy sunlight-gathering systems as a way to conserve. And is it ever beautiful!”

Fascinated and, feeling a little tired from the high-energy Nye Guy, I felt content in fully understanding this ‘Circle of Tree Life’.

Check back tomorrow as we take this leaf-falling phenomen a step further and come up with some great ways to use those leaves around your yard and home this fall and winter.

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