Goodbye, Leaves
Summer has flown, Labor Day has passed, and we are now looking at the seasonal changes of nature preparing itself for winter. Not noticeably, but trees have already been readying themselves for cold weather since mid-summer. As we know, the leaves on trees will change colors soon and eventually fall off. However, there is another physiological change in trees that precedes leaf color and fall. Leaves do not just die and fall off the tree. The tree cuts them from the tree and then pushes them off.
This cutting off process is known as abscission (from the Latin root “to cut”) Abscission, in botanical terms, describes the process by which a plant drops one or more of its parts (a leaf, flowers, ripe fruit, seeds, etc.).
Abscission in a tree’s leaf takes place in the area where the leaf stem, or petiole, meets or is attached to the tree or twig. This layer of connective tissue in the leaf stem is called the abscission zone and begins developing as the leaf sprouts in spring. This zone contains smaller, weaker, fibreless cells that grow to be easily broken and then to seal the broken scar.
As the abscission process begins, two microscopic layers of cells form in the petiole base. The first layer to form is the separation layer which contain short cells with thin walls. As cooler weather with less sunlight approaches, this separation layer grows, causing the petiole to begin separating from the tree or branch. Eventually, the weight of the leaf will cause it to fall, especially in rain or a windstorm.
A second layer of fatty tissue also forms between the cells on the stem side of the separation layer. As the leaf begins separating, this layer then seals off the opening on the branch to prevent water loss from the tree and an to keep insects and other health issues from entering the tree.
The tree cuts off leaves in this manner to protect the tree during dormancy in the winter. This abscission layer forms the leaf scar that can be seen at the end of bare branches and twigs in winter. A closer look at the leaf scar may reveal tiny dots that are the ends of veins which previously carried nutrients and water to the living leaf.
Leaf abscission is one of the tree’s physiological functions necessary for the tree’s survival. It is a very microscopic function, but important part of the many interest ways a tree grows.
If you have questions or concerns, feel free to contact the Vermilion Tree Commission by contacting Anne Maiden at the Mayor’s Office at annemaiden@vermilion.net or at 440-204-2402. You are also cordially invited to attend our monthly Tree Commission meeting which is on the second Wednesday of the month. The Tree Commission will meet at the Ritter Public Library on Wednesday, September 13, at 7:00 PM
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