Sometimes I like to go to my happy place. This
time, it was so that I could relax and enjoy my happiness at being a new
grandfather. For some people, their happy place is a fantasy inside their
minds. But for me, it is an actual place: the forest. In particular, the forest
on Turkey Mountain outside of Tulsa.
The forest is a place with layers upon layers
of stories. It is a palimpsest, that is, a document with layer upon layer of
writing. On this particular like, I already knew almost all of the stories. One
story was the geological history of the landscape. As I looked down from the
mountain onto the Arkansas River, I remembered that this mile-wide river was
once ten miles wide, as the glaciers up north melted. Another story was about
human impact. The forest all around me grew in just the last century, since the
time when this mountain was covered with oil derricks. Each species of tree,
and each tree, had its own story: an evolutionary story of adaptation, and a
developmental story of how each individual tree adjusted to its immediate
environment of shade and sun, of deep soil or rock outcrop. The story is
written from moment to moment. When I hiked, a drought was underway, and many
leaves were wilted. Wilting is not completely a bad thing; by drooping down, a
leaf has less of a heat load from the sunlight and avoids some of the damage it
would otherwise experience. There are also many stories of food webs, whether
it is of insects that have eaten the tender leaf tissue between the veins, or
fungi eating dead branches. There is the story of the hard, green, inedible
persimmon fruits that keep animals away until the seeds are ready to disperse,
and only then do the fruits become ripe and delicious. There is the story of
stumps sprouting back to life.
I knew all of these stories, but I just wanted
to see them again on this hike. But I kept my eyes open, just in case there was
a new story. And I found one—just one. A mimosa sampling was hit hard by the
drought. Mimosas have lots of little leaflets which, after being held
horizontally to face the sunlight in the day, close upward at night for reasons
that nobody knows for sure. The leaflets also close upward during moderate
drought, thus reducing their solar heat load. The leaflet movements are caused
by bags of water called pulvini (singular pulvinus). When the bags of water
exert pressure, they push the leaflets; when the bags lose pressure, the
leaflets return to their original position.
But what I did not know was, are the pulvini on
the top of the leaf or the bottom? If they are on top, then the water pressure
pushes the leaflets open; their nocturnal closure represents only a loss of
pressure, a relaxation, going to sleep. If this is the case, then the closure
of the leaflets at night does not need an explanation. The plant is saving
energy by letting the leaves relax. But if the pulvini are on the bottom, then
the nocturnal closure demands an explanation: why would the mimosa leaf deliberately close its leaflets at
night? This requires the expenditure of energy. The same is true for leaflet
closure during moderate drought. If the pulvini are on the top, leaflet closure
is merely the result of drought; but if they are on the bottom, then leaflet
closure is a deliberate act to prevent drought damage. Of course, the pulvini
are too small for me to see.
But I saw one leaf that was so stressed by heat
and drought that the leaflets were starting to curl up and die. And in this
leaf, the leaflets were horizontal. This tells me that the pulvini are on the
bottom. I know this because, when severe drought causes the pulvini to stop
working, the leaflets are horizontal, not vertical. In the photo, the drought-damaged leaflets are on top, the leaflets that are actively responding to drought are on the bottom. The damanged leaflets are horizontal, and the responsive leaflets are almost vertical.
To me, this was a new story that I learned on
my hike. But I enjoyed re-experiencing the thousand other stories in the forest
that I already knew.
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