The part of autumn I want to look at is smaller than a leaf. It is the place on a twig where a leaf used to be.
A bare branch can look unfinished, as though the wind has stripped something away. Look closely at the attachment point, though, and there is a distinct mark. The leaf did not simply tear loose wherever its stalk happened to be weakest. The tree has a particular place for this separation.
Botanists call the process abscission. The remaining mark is a leaf scar.
Where the stalk lets go
During summer, a leaf exchanges water and nutrients with the rest of the tree through its stalk. Near the stalk’s base is an abscission zone, a small region whose cells differ from their neighbors even before autumn’s changes become visible.
Bob Thomas’s explanation for Loyola University’s Center for Environmental Communication follows what happens there as days shorten. Chemical changes produce two distinct layers with different jobs.
The separation layer has short, thin-walled cells. It becomes a weak connection. The leaf’s own weight, sometimes helped by wind or rain, can then finish the break.
On the twig side, a protective layer forms as fatty material accumulates within and between cells. The departure leaves a protected attachment site rather than a fresh, ragged tear.
That changes how I picture a windy October afternoon. The gust may supply the final pull, but the preparation happened in living tissue. A leaf can fall because the tree has changed the connection holding it up.
Read the little dots
The scar can contain smaller marks of its own. Thomas describes these as bundle scars, the ends of the veins that once carried nutrients to the leaf. Under a close look, their arrangement can resemble a tiny face.
I like the specificity of that. A branch loses a broad green blade, yet keeps a visible trace of the connections that supplied it.
For a closer look, find an already bare twig on a tree you can examine without breaking a branch. Look for the mark where a stalk met the wood. Then see whether you can distinguish dots inside it. A magnifier may help. There is no need to pull off a leaf to make the demonstration happen.
The colorful part of the season gets plenty of attention. Here is something to inspect after it is gone.
Some leaves stay attached
A brown leaf still hanging in winter complicates the picture. Oaks and American beeches can retain dead leaves, especially on young trees or lower branches. The name for that is marcescence.
University of Maryland Extension forester Andrew Kling explains that the separation is delayed until spring. He also makes a useful distinction between naming the process and explaining its advantage. Proposed benefits include protecting buds from browsing animals or delivering leaf litter in spring, but the reason is not settled.
So a winter branch offers two things to look for. There may be a leaf scar to examine, or a dry leaf whose attachment has lasted longer. Either is worth stopping for. I am particularly taken with the dots, a little piece of last summer’s plumbing left in view.
Moo for now,
Maude 🐄