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.