While walking to lab last week, a stunning sight caught my eye. It was a young gingko tree, covered in bright green-yellow leaves. This tree was interesting to me because I knew that all other tree of this species have dropped their leaves by this point in the season. Their fan-shaped leaves had fluttered silently to the ground, scattered about and collected in gutters. This tree, however, was as full as could be.
Immediately, I wanted to know more about why I found this tree so odd.
Left: Anatomy of a leaf. Right: Botanical illustration of Ginkgo biloba.
The gingko tree (Ginkgo biloba) is absolutely ancient, with its earliest fossils dating back to 270 million years ago. Perhaps because of its age, this species is an oddity in that it has no no known living relatives. Specifically, the gingko trees is so anomalous that it has been classified into its own division, Ginkgophyta, with a single class, order, family, genus and species, of which Ginkgobiloba is the only extant representative. The species is both deciduous (meaning it loses its trees in winter) and coniferous (meaning it bears cones).
The primary role of a leaf is to produce food for the tree. Therefore, the structure of a leaf has adapted to best absorb light energy from the sun in order to conduct photosynthesis. Photosynthesis is carried out by the chloroplasts found within the leaf cells. The reason why many leaves are so thin (those of the gingko tree included) is because light must be able to penetrate all of the cell layers. Having picked up a leaf from the ground, I decided to place a bit on a slide to examine. As shown in the images below, I was able to make out translucent, circular shapes, some of which were tinted green. I could also make out thin green veins running vertically across the sample.
As the weather shifts and days get shorter, deciduous drop their leaves by a process called absession, when specialized cells sever the leaf from its branch. By shedding its leaves, trees are able to conserve water and energy through days of less sun and chilly weather. During the absession process, the tree absorbs nutrients from the leaves to store in their roots for future use. Chlorophyll, the compound that enables photosynthesis and gives leaves their green color, is one of the first to break down and is the reason leaves in the fall change color before dropping. I wonder if the clear cells in the images above were filled with green chlorophyll earlier in the year.
I also wondered why this tree in particular took so long to drop its leaves. I noticed that it’s exposed location in the plaza granted it a great deal of sunlight, whereas other trees on the campus would have been shadowed by surrounding trees or buildings. One source I found online wrote that the gingko has been observed to drop its leaves all at once, after the first hard frost. The temperature in Princeton this fall has been pretty mild, but I know that the past few days have been below freezing, so I took a walk to the gingko tree.
Some leaves had fallen! And the green tinge I noticed last week was all but gone. I am surprised by the tenacity of this tiny, ancient tree.
I conducted this project as part of Professor Pringle’s EEB321 class at Princeton University.
The Life Story of The Oldest Tree on Earth. Yale E360 https://e360.yale.edu/features/peter_crane_history_of_ginkgo_earths_oldest_tree.
Plant Life: Leaf Anatomy. http://lifeofplant.blogspot.com/2011/03/leaf-anatomy.html.
Conifer Cousins: Ginkgo biloba. American Conifer Society https://conifersociety.org/conifers/articles/conifer-ginkgo-biloba/.
Abscission. Biology Articles, Tutorials & Dictionary Online https://www.biologyonline.com/dictionary/abscission (2019).